A written corpus of genealogies was maintained in Ireland from at least the seventh century. Though the corpus is transmitted to us in genealogical collections of the twelfth century or later, the texts contained within these manuscripts contain archaic language and substrata that indicate that the corpus was regularly updated from earliest times. We are extremely fortunate in that the Cenél nEoghain genealogy is one of the most fully recorded within the corpus and that the keeping of annals began circa 550 on the island of Iona under Saint Colmcille. The Iona Chronicle is heavily concerned with the northern Uí Néill kinsmen of Colmcille as is its continuation which was maintained in the north of Ireland from circa 740. Through various routes of transmission these have supplied the authentic early stratum lying behind all of the extant collections of Irish annals (eg: Annals of Ulster, Annals of Inisfallen, Annals of Tigernach) and this has left the Cenél nEoghain genealogy as one of those most amenable to authentication within the corpus.
The Iona Chronicle and its continuation are best exemplified in the Annals of Ulster which is the product of a compilation of several closely related strands in the eleventh century. By adopting a conservative methodology and cross referencing the Cenél nEoghain genealogy only to Annals of Ulster entries that have been shorn of all known or suspected interpolation (eg: those written in a later hand in the surviving manuscript, those that betray knowledge of subsequent events, those that are duplicated or are drawn from just one strand indicating insertion during transmission) and which conform to its linguistic gradient (earlier>later : Latin>Irish) and its stylistic gradient (earlier>later : less detail>more detail) we are in a position to authenticate the genealogy by reference to annals that have every appearance of being contemporaneous. It is not known when the entries prior to circa 550 were inserted into the annals so we are advised to carefully analyse every such entry on which we seek to rely and evaluate it strictly on its own merits. The death entry of Eoghan son of Niall reported in 465:
and so may be admitted to the authentication. Unfortunately the entry finds no parallel corroboration in the Annals of Tigernach due to a gap in the surviving text or the Annals of Inisfallen which have suffered abbreviation but this is in the nature of these early sources and so does not render the entry any less reliable a witness. Much the same argument can be made for the admission of the somewhat less laconic death entry of Muircheartach reported in 534 except that this time the entry finds parallel corroboration in the Annals of Tigernach in 532 and so clearly forms part of the authentic early stratum lying behind all of the extant collections of Irish annals. On this basis the authenticated lineage of Lochlainn from the fifth century to the ninth century is as follows :
Niall Noígiallach, son obit 465 | Eoghan, obit 465 | Muireadhach | Muircheartach Mac Ercae, obit 534 | Domhnall Ilchelgach, obit 566 | Aedh Uaridnach, obit 612 | Maolfithrich, obit 630 | Maolduin, obit 681 | Fearghal, obit 722 | Niall Frossach, obit 778 | Aedh Oirdnide, obit 819 | Niall Caille, obit 846 | Aedh Findliath, obit 879
The genealogical collections proceed to give Aedh Findliath two sons and at this point the genealogy of Clann Néill diverges from that of Clann Domhnaill. We know from a reading of the annals that Clann Néill were in control of Tullaghoge and the south of Tír nEoghain and bore the surname O Neill while Clann Domhnaill were in control of Inishowen and the north and seem not to have borne a surname. The genealogical collections show the MacLochlainns of Inishowen originating in the following eleventh century lineage :
Lochlainn | Ardghal, obit 1064 | Domhnall, obit 1121
A problem presents itself at this point as the genealogical collections bridge the gap between Aedh Findliath and Lochlainn with two alternative lineages (one through Clann Néill and one through Clann Domhnaill) rendering Lochlainn's lineage through the tenth century controversial. Ardghal died in 1064 so we might expect his father to have flourished in the early eleventh century. The only member of the Cenél nEoghain named Lochlainn to be mentioned in the annals around that time appears in 1023 in a death entry given most fully in the Annals of Tigernach of a Lochlainn who was ruler of Inishowen and Magh Iotha with father Maelsechlainn and brother Niall. Maelsechlainn has an alternate spelling Maelsechnaill  and so we can construct the following mini-genealogy from the entry:
Maelsechlainn/Maelsechnaill |__________ | | Niall Lochlainn, obit 1023
When the genealogical collections are augmented with this mini-genealogy they indicate that there may be two Lochlainns each with a single lineage back to Aedh Findliath rather than a single Lochlainn with two alternative lineages. In the analysis that follows the ancestor of the MacLochlainns of Inishowen is referred to as Lochlainn A (A for ancestor) while the ruler of Inishowen and Magh Iotha is referred to as Lochlainn R (R for ruler) allowing the question to be formulated as follows. Are Lochlainn A and Lochlainn R two people or are they two aspects of the same person?
The Cenél nEoghain genealogy in the fifteenth century manuscript Laud 610  is a copy of a compilation made circa 1050 and terminates at a point at which the identification of Lochlainn R can be inferred but is silent as regards Lochlainn A:
Aedh Findliath |____________________________ | | Domhnall Niall Glúndubh | | Flann Muircheartach : | : Domhnall : | Maelsechnaill Muireadhach |__________ | | Niall Lochlainn R
Laud 610 is in normal text and the inference made is highlighted. Note the discontinuity between Flann and Maelsechnaill which is bridged by the descriptor diatá ('as a consequence exists'). The Clann Néill and Clann Domhnaill genealogies in the twelfth century manuscript Rawlinson B502  compiled circa 1120 are the earliest genealogical source in which Lochlainn A can be identified:
Aedh Findliath |____________________________ | | Domhnall Niall Glúndubh | | Flann Muircheartach | | Maelruanaid Domhnall | |___________________ | | | Maelsechnaill Muireadhach Muircheartach |_________ | | | | | | Niall Lochlainn R Lochlainn A Flaithbhertach | | | Aedh Ardghal Niall | | Domhnall Aedh
Rawlinson B502 is in normal text and the inferences made are highlighted. Lochlainn A is clearly a separate person to Lochlainn R. Note that the Clann Néill and Clann Domhnaill genealogies flow straightforwardly from the earlier genealogy of Laud 610. Note also that the lineage of Aedh son of Niall on the far right appears in the manuscript as an alternative lineage under the Latin header item ('also') immediately following the Clann Domhnaill lineage ascending from Aedh son of Niall on the far left. Taken together with the discontinuity between Flann and Maelsechnaill in Laud 610 we can see that the Clann Domhnaill genealogy is somewhat uncertain even at this early stage with the Clann Domhnaill representative Aedh son of Niall having an alternative ascent through Flaithbhertach (clearly the prominent Cenél nEoghain ruler Flaithbhertach an Trosdáin O Neill who died in 1036) rather than Clann Domhnaill! The King of Aileach genealogy in the twelfth century manuscript Book of Leinster  compiled circa 1170 is the earliest genealogical source that allows us to identify Lochlainn A and Lochlainn R as the same person:
Aedh Findliath | Domhnall | Flann | Maelruanaid | Maelsechnaill | Lochlainn A/R | Ardghal | Domhnall | Niall | Muircheartach
The Book of Leinster is in normal text and the inferences made are highlighted. Note that the King of Aileach genealogy does not flow straightforwardly from the earlier genealogies of Rawlinson B502 or Laud 610.
So which is correct, Rawlinson B502 with a Clann Néill ancestry for Lochlainn A or the Book of Leinster with a Clann Domhnaill ancestry for Lochlainn A/R? The remaining genealogical collections are of limited use as they preserve compilations of a much later date but contemporaneous annals do provide some illumination. Many entries in the annals provide a patronymic, a mini-genealogy in the form X mac Y ('X son of Y'), X ua Y ('X grandson of Y') or occasionally a longer string such as X mac Y meic Z ('X son of Y son of Z') though that patronymics transform into surnames when not literally true and that a genealogy is required to determine the literal truth of a patronymic is a circularity that should always be borne in mind when parsing these materials. Note in particular that the (personal name)(patronymic) formula used to identify an individual in terms of genealogy begins to be displaced in the tenth and eleventh centuries by the (personal name)(optional patronymic)(surname) formula that identifies an individual in terms of surname.
The Annals of Tigernach in 1064 identify Ardghal as Ardghal mac Lochlainn ua Néill. O Neill is a well attested surname within the Cenél nEoghain at that time and so the identification would appear to be of (personal name)(patronymic)(surname). By 1064 the Annals of Tigernach are contemporaneous so as the entry survives within the main text of a late fourteenth/early fifteenth century copy and there is nothing to indicate that it was interpolated it is a good witness that Ardghal bore the surname O Neill as a member of Clann Néill consistent with Rawlinson B502.
The Annals of Inisfallen in 1093 identify Domhnall in the office of king as mac meic Lochlainn ua Maelsechnaill. O Maelsechnaill is unattested as a surname within the Cenél nEoghain at that time and so the identification would appear to be of (personal name)(patronymic) with personal name omitted as superfluous in view of the high office of the individual concerned. By 1093 the Annals of Inisfallen are contemporaneous so as the entry survives within the main text of the original manuscript it is a good witness that Domhnall was grandson of Lochlainn A as grandson of Maelsechnaill. While the 1093 entry is inconsistent with Rawlinson B502 (there is no Maelsechnaill in the line of Lochlainn A) it is significant that it is also inconsistent with the Book of Leinster which gives Maelsechnaill as father rather than grandfather of Lochlainn A.
The Annals of Ulster in 1099 identify Domhnall as both Domhnall ua Néill and Domhnall ua Flainn. This entry survives in one late fifteenth century copy and one early sixteenth century copy of this copy but the portion of interest to us, a eulogistic verse, appears only as a marginal entry in a secondary hand in the first of these indicating that the verse was a late interpolation not present in the original contemporary annal so making it a poor witness. That the 1099 verse is consistent with both Rawlinson B502 (giving Domhnall as a member of Clann Néill the well attested surname O Neill) and the Book of Leinster (giving Domhnall as great-great-great grandson of Flann the surname O Flainn otherwise unattested within the Cenél nEoghain) would seem to confirm this assessment leaving open the possibility that the verse is itself an attempt to reconcile the materials that we are considering. Given these concerns the 1099 verse is best excluded from further consideration as is its partially silenced analogue in the seventeenth century compilation Annals of the Four Masters.
I can offer no reason why the Annals of Inisfallen is inconsistent with both Rawlinson B502 and the Book of Leinster beyond its spatial distance as a Munster chronicle from the northern events that it describes and its temporal distance (four generations = one hundred and forty years) from the furthest extent of the unusually lengthy patronymic mini-genealogy it ascribes to Domhnall. On a methodological level we should remember that annals are not genealogies and so it would be unwise to admit the identification by genealogy of the Inisfallen annalist as a third way against Rawlinson B502 whereas the identification by surname of the Tigernach annalist is clearly within his competence and so can be fully admitted in support of Rawlinson B502. The unavoidable conclusion is that the evidence in support of the Book of Leinster genealogy is the weaker being not just a later genealogical source than Rawlinson B502 but being contradicted by every other source. Rawlinson B502 on the other hand is not just an earlier genealogical source but is supported by the earliest annalistic source the Annals of Tigernach. The weight of direct evidence clearly indicates that in the matter of the ancestry of Lochlainn Rawlinson B502 is to be preferred over the Book of Leinster leaving us with two Lochlainns rather than one.
The question of Lochlainn’s ancestry has received the attention of several academics over the years yet none of them seems to have found the basic inconsistency between the Book of Leinster and the Annals of Inisfallen to be worthy of comment. In a cumulative case arguing from the silence of the annals as to a Lochlainn other than Lochlainn R as if absence of evidence was evidence of absence, their silence as to his father Muireadhach despite his mention in the Annals of Ulster in 1015 and considering much late and irrelevant material they imagined that the argument was evenly matched with the Book of Leinster being supported by the Annals of Inisfallen and went on to develop some additional arguments to tip the balance. Though now rendered redundant I will examine each set of arguments in turn. None of them are particularly convincing. In fact all are remarkably flawed.
James Hogan  asserted that the MacLochlainns were associated with Inishowen since the time of Lochlainn R indicating that Lochlainn A and Lochlainn R were the same person. He offered no evidence for his assertion which would appear to be contradicted by the annals which disclose no such association and in particular by the Annals of Ulster which show that Ardghal was ruler of Tullaghoge in 1051 and died in Tullaghoge in 1064. Hogan also argued that the marriage of Domhnall to the daughter of a usurper of the Clann Néill rulership of Tullaghoge was evidence that Domhnall was a member of Clann Domhnaill. The argument rests upon an assumed date of marriage, however, and shift the assumed date by just a few years and it is just as easily recast as a post-usurpation alliance between Domhnall and the enemies of his rival for the kingship of Ireland.
Séamus Ó Ceallaigh  seemingly on the basis of an unsustainable presumption that one surname (MacLochlainn) cannot emerge from another (O Neill) asserted that every entry relating to Domhnall in the Annals of Ulster except for the 1099 entry previously discussed are evidence that he was of Clann Domhnaill. But these entries refer to him as either descendant of Lochlainn, grandson of Lochlainn or son of Ardghal son of Lochlainn and so clearly provide no evidence one way or the other. Ó Ceallaigh went on to construct a circular argument in order to conclude that alliances recorded in the annals between the men of Magh Iotha and Ardghal in 1053 and the men of Magh Iotha and Domhnall in 1080 flowed from the relationship that Ardghal and Domhnall enjoyed with Magh Iotha as descendants of Lochlainn R. This is a very rash presumption given the rapidly shifting loyalties that were a feature of contemporary dynastic politics and so there seems little basis for his conclusion beyond its circularity.
Donnchadh Ó Corráin  attempted to remove the contradiction in the 1099 entry in the Annals of Ulster by re-interpreting the text so that Domhnall becomes a descendant of Flann and Niall [Noígiallach] rather than Flann and Niall [Glúndubh]. As Flann is himself a descendant of Niall Noígiallach Ó Corráin declares the entry in full support of the Book of Leinster then goes on to suggest that the genealogy in Rawlinson B502 was faked in order to confer a more glorious ancestry upon the MacLochlainn dynasty. We have seen above, however, that the verse in question is a late marginal interpolation and so its re-interpretation clearly offers us nothing of value at this point. Of the faking allegation, moreover, all that can be said is that it would appear to be nothing but an unsupported conspiracy theory by which the contrary evidence is wished away.
Francis J Byrne  asserted that the 1064 entry in the Annals of Tigernach was characteristic of a genealogical gloss silently interpolated into the main text of the annals so reducing its value as a witness. He does not elaborate upon his assertion, however, which would seem to leave it as yet another conspiracy theory but when we work through the scenario we see that the 1064 entry harmonises with the Rawlinson B502 genealogy only when interpreted as (personal name)(patronymic)(surname) and so the genealogical content (the patronymic mac Lochlainn) is irrelevant even if it were silently interpolated because the remainder of the entry still witnesses that Ardghal bore the surname O Neill as a member of Clann Néill consistent with Rawlinson B502. The assertion of Byrne would thus appear to be unsustainable.
A New Theory
Having dealt with these arguments one important question still remains. Why does the Book of Leinster contradict Rawlinson B502? We should always be open to the possibility of medieval Irish genealogies being reinventions of the past with the macro-relationships between lineages being periodically recast in order to validate the status quo by projecting it unchanged into the distant past . I would suggest that the Book of Leinster genealogy is the product of just such a construct created in order to underpin an important territorial settlement that occurred in the interim between compilation of the Rawlinson B502 genealogical collection circa 1120 and compilation of the Book of Leinster genealogical collection circa 1170. From a reading of the annals the lineage that gave rise to the MacLochlainn dynasty migrated northwards between the death of Ardghal in Tullaghoge in 1064 and the death of Domhnall in Derry in 1121. Clann Domhnaill fade from view during this time and the MacLochlainn dynasty become paramount across Tír nEoghain until 1167 when the north/south split is reimposed by the O Connor dynasty of Connacht. According to the Annals of the Four Masters in 1167:
ro rann Ua Conchobhair an tír i ndó i Tír Eoghain o Shléibh Challain fo thuaidh do Niall Ua Lachlainn dar cend da bhrághadh i Ua Catháin na Craoibhe 7 mac An Ghaill Uí Bhrain 7 Cenél Eoghain ó Shlebh fo dheas do Aedh Ua Néill dar cend dá bhrághatt oile i Ua Maoil Aedha do Chenél Aonghusa 7 hUa hUrthuile do hUibh Tuirtre
'O Connor divided the territory into two parts, that is Tír nEoghain north of Slieve Gallion to Niall MacLochlainn for two hostages [named] and south of the mountain to Aedh O Neill for two other hostages [named]'
If we tabulate the reigns of the territorial rulers appearing on a terminal generation in the particular Book of Leinster genealogical tract in which the King of Aileach genealogy appears we find that with the exception of Connacht the reigns overlap only within the range 1152-1153:
Ulster 1131-1157 Meath 1152-1155 Scotland 1124-1153 Desmond 1143-1185 Thomond 1118-1167 Leinster 1126-1171 Ossory 1151-1162 Aileach 1136-1166 Airgialla 1127-1168 Connacht 1156-1183 Breffni 1128-1172
This indicates that the Book of Leinster genealogical tract was in origin an O Connor of Connacht document produced between 1152 and 1153 to which the genealogy of the currently extant O Connor has been added by the Book of Leinster compiler. I would suggest that the MacLochlainn genealogy contained within the document was created in order to legitimise a territorial settlement agreed in advance between O Connor and the O Neill opposition within Tír nEoghain. The MacLochlainn dynasty were to be restricted to Inishowen and the north so they were made to descend from the by then obscure Clann Domhnaill of Inishowen by conflating Lochlainn A and Lochlainn R. The O Neill opposition were to be given Tullaghoge and the south so they were made to descend from Clann Néill of Tullaghoge by splicing them vertically onto the prominent Cenél nEoghain ruler Flaithbhertach an Trosdáin O Neill. This created a sideways shift in the genealogies as they were first brought into line with the proposed settlement then pressed into service by incorporation within the Book of Leinster to confer legitimacy upon it. In summary the MacLochlainn genealogy was recast by O Connor for political purposes as follows:
Aedh Findliath |_________________ | | Domhnall Niall Glúndubh |Clann Domhnaill |Clann Néill Flann Muircheartach | | Maelruanaid Domhnall | |___________________ | | | Maelsechnaill Muireadhach Muircheartach |<----------------| | Niall Lochlainn Flaithbhertach | | |<----------------| Aedh Ardghal Niall Aedh | | | Domhnall Aedh Domhnall | | Niall Flaithbhertach | | Muircheartach Conor | | Niall MacLochlainn Tadhg Inishowen 1167 | Muircheartach | Aedh O Neill Tullaghoge 1167
Rawlinson B502 is the precursor and is in normal text. The shifting territorial position to be accounted for within the artificial construct and the generations accruing subsequent to Rawlinson B502 are highlighted. The sideways shift is shown as <-----------
Aedh Findliath |_________________ | | Domhnall Niall Glúndubh | | Flann Muircheartach | | Maelruanaid Domhnall | |___________________ | | | Maelsechnaill Muireadhach Muircheartach |<----------- | Lochlainn Flaithbhertach | |<----------- Ardghal Aedh | | Domhnall Domhnall | | Niall Flaithbhertach | | Muircheartach Conor | | Niall MacLochlainn Tadhg | Muircheartach | Aedh O Neill
The MacLochlainn genealogy accruing subsequent to Rawlinson B502 can be authenticated from entries in the annals but the O Neill genealogy cannot so I would contend that the O Neill genealogy accruing subsequent to Rawlinson B502 and the conflation of Lochlainn A and Lochlainn R are an artificial construct of which the Book of Leinster genealogy highlighted is a product. On a methodological level this new theory dispenses with the need for a seperate theory addressing the other major problem in Cenél nEoghain genealogy (the unauthenticated and overlong O Neill genealogy ) and so is to be preferred for its economy leaving Lochlainn A firmly identified as a member of Clann Néill with an authenticated lineage back to the fifth century as follows :
Niall Noígiallach, son obit 465 | Eoghan, obit 465 | Muireadhach | Muircheartach Mac Ercae, obit 534 | Domhnall Ilchelgach, obit 566 | Aedh Uaridnach, obit 612 | Maolfithrich, obit 630 | Maolduin, obit 681 | Fearghal, obit 722 | Niall Frossach, obit 778 | Aedh Oirdnide, obit 819 | Niall Caille, obit 846 | Aedh Findliath, obit 879 | Niall Glúndubh, obit 919 | Muircheartach, obit 943 | Domhnall, obit 980 | Muireadhach, son obit 1015 | Lochlainn
As a final point I would note that the ancestor of choice among the twelfth century Cenél nEoghain is clearly the prominent eleventh century ruler Flaithbhertach an Trosdáin O Neill having been appropriated as an ancestor by both Clann Domhnaill and the O Neill opposition that came to power in 1167. The authenticated MacLochlainn genealogy makes no such illustrious claim but proceeds instead through the relatively obscure and non-regnant Muireadhach for little apparent gain so one might now hope that even conspiracy theories holding that the MacLochlainn genealogy was faked in order to confer a more glorious ancestry might finally be laid to rest.