The Grianan of Aileach is a circular stone fort crowning the westernmost peak of the heights dominating the isthmus leading onto the Inishowen peninsula. The structure is encircled by three earth ramparts, contains numerous passages, staircases and terraces within the thickness of its massive single wall and is thought to date from the Iron Age. In the twelfth century body of onomastic tradition known as dinnshenchas ('place-lore') the structure is said to have been built upon a burial site in the time of an ancient people known as the Tuatha Dé Danann who were possessed of magical powers . Modern historians believe the Tuatha Dé Danann to be artefacts of the gods and goddesses of the old pre-Christian religion reduced to mortality so as to accord with Christian sensibilities by seventh century ecclesiastical schools as they fabricated a pseudohistory projecting into the distant past known as Lebor Gabála Érenn ('Book of Invasions of Ireland'). So while we should not regard this tradition as being historical it could well be the case that the structure was built upon the site of a Bronze Age tumulus whose remains were still visible in the twelfth century. What we do know is that the Grianan of Aileach first comes to the notice of history as a distinct structure in 676 as Aileach Fringrenn ('Aileach of Frigriu') within the earliest stratum of the Annals of Ulster . According to the twelfth century dinnshenchas Frigriu was one of the builders of the structure, though in the way of such tradition this is likely a fanciful rationalisation of a meaning that had long become obscure. The modern placename Grianan of Aileach is an anglicisation of a placename attested from the twelfth century  formed from the elements gríanán ('solarium', 'summer-palace') and the name of the district (Aileach, 'Stony Place') within which the structure lies. With the name of the district declined into genitive form the placename becomes Gríanán Ailigh ('Solarium/Summer-palace of Aileach'). The seasonal occupation implied by its name and inhospitable position would place the structure in constellation with winter quarters that can be posited to have existed at an intervisible high status site upon an irregular rocky outcrop now occupied by the remains of a fourteenth century enclosure castle at Elagh (an anglicisation of Aileach) in the fertile southern lowlands of the peninsula.
The ninth century hagiography Bethu Phátraic ('[Tripartite] Life of Patrick') narrates a circuit said to have been made by the Christian missionary Saint Patrick around the northern half of Ireland in the fifth century. Of present interest it contains an account telling how Eoghan son of Niall Noígiallach went to meet Patrick at a place now unknown called Fid Mór ('Great Boundary Tree') near the Grianan of Aileach. The account (at least in this final form) would appear to be a work of propaganda by Comharba Phátraic ('Successor of Patrick') seeking to legitimise the political claims of his allies the Cenél nEoghain by imputing certain acts and prophecies to the saint. In the account Eoghan leaves the Grianan of Aileach to meet Patrick at the boundary of his kingdom and during their conversation complains of his own ugliness. Patrick covers Eoghan and a handsome youth with a sheet whereupon they fall asleep so that when Eoghan awakes he finds that he has acquired the handsome looks of the youth. Eoghan then complains of his lack of height. Patrick asks Eoghan to indicate his desired height with an outstretched arm whereupon Eoghan immediately grows to that height. These marvels complete they then proceed to the Grianan of Aileach where Eoghan is baptised a Christian. On departing to continue his journey Patrick leaves a flagstone and prophesies that kings of Aileach will rule over Ireland . The details are clearly apocryphal being typical of the legends attached to the early saints in hagiographical literature. In this case they serve to bolster the Cenél nEoghain by indicating that they enjoy the favour of God. The basic framework of the account would, however, appear to be more robust. An Eogan i Fid Mór ('Eoghan at Great Boundary Tree') incident is referenced in eighth century notulae ('brief notes') contained within the Book of Armagh that act as an index to a lost collection of early traditions relating to Patrick  and so the account can be seen to reflect a more authentic early tradition of which we can say no more than it concerned a meeting between Eoghan and Patrick that took place upon a boundary marked by a tree.
In medieval Ireland a person was inaugurated into kingship by way of a ceremony during which he stood upon a flagstone or sat upon a stone chair that was consecrated to that purpose. Whatever its provenance we might expect to find such an object at a royal site as important as the Grianan of Aileach. No such flagstone or stone chair has yet been found within its precincts but a small tumulus now barely visible between the outer earthen banks could well be the place where such an object once lay. Three miles away in the northern suburbs of modern Derry lies a flagstone commonly known as Saint Columb's Stone. Lying next to a mound known as Saint Columb's Mount and having the impression of two feet left and right sculpted into its upper surface it is strongly suggestive of a flagstone upon which a person would stand while being inaugurated into kingship. The Grianan of Aileach was seriously damaged during an incursion by the O Brien dynasty of Munster into the north in 1101 and thereafter seems to have been abandoned as a royal residence in favour of Derry so it may be that Saint Columb's Stone was moved from the Grianan of Aileach closer to Derry at that time. As for the Grianan of Aileach it would seem to have been left to quietly deteriorate in its remote position until attracting the interest of antiquarians in the nineteenth century . The upper reaches of its walls have since been rebuilt from fallen masonry and it is now in the care of the Irish state.