That the vast _dun_, or enclosure, on Knock Aillinn was an ancient residence of the Kings of Leinster is generally admitted; and that it was erected long previous to the Christian era is also the opinion of those best acquainted with early Irish history and literature. Proofs of this can be obtained from the most reliable and ancient Gaelic writings. There is hardly a vestige of antiquity to be seen on the summit of Knock Aillinn save the vast earthen rampart. When one stands within it, and recalls to mind what it must have been in days long gone by, when a large population dwelt in it, and when armed multitudes issued from it, he will be tempted to exclaim with Byron:-- "Shrine of the mighty! can it be That this is all remains of thee?" He will wonder that no vast masses of ancient masonry are to be seen. But stone buildings of the kind that have been in use in these islands for nearly a thousand years were unknown when the vast earth-works on Knock Aillinn were erected. Walls built of dry stone have been used in Ireland as fortresses from the most remote antiquity; but the art of building with mortar was entirely unknown until after the introduction of Christianity. The hill of Allen is the one on which, it is over and over again stated by the most ancient and trustworthy Gaelic documents extant, Finn, the son of Cumhail, had his palace. We are even told how, partly by force and threats, he obtained Allen from his grandfather, Tadg; that he went to live on it, and that it was his habitation as long as he lived. But here a great difficulty meets us--there is not a vestige of dun or fort on the hill of Allen. O'Donovan says in his unpublished letters, while on the Ordnance Survey of Ireland, that Knock Aillinn was, according to various ancient Irish authorities, one of the royal residences of the Kings of Leinster, and that it received the name of _Aillinn_ from the _ail_, or stone which was placed in the mound of the rath. On speaking of the hill of Allen, where the celebrated Finn Mac Cool or Cumhail is said to have had his seat, he says, "There are no traces of forts nor any other monuments excepting one small mound called _Suidhe Finn_, or Finn's chair, which occupies the highest point of the hill. On every side of this mound there are faint traces of field works, but so indistinct that I could not with any certainty decide whether they are traces of forts or of recent cultivation, for the hill was tilled on the very summit. I travelled all the hill, but could find upon it no monument from which it could be inferred that it was ever a royal seat like Tara, Emania, Maistean, or any of the other places of ancient celebrity whose localities have been identified; and still in all Fingallian or Ossianic poems this hill (the hill of Allen) is referred to as containing the palace of the renowned champion, Finn Mac Cool, who seems to have been a real historical character, who flourished here in the latter end of the third century."