Illan was a traditional king of Leinster, a province in Ireland, who was thought to have conducted raids in Britain. J. Morris argues that he would have been one of the historical Arthur’s enemies. The traditional dates of Illan’s reign are AD 495-511, but the history of Leinster at this period is obscure and Illan may have reigned at an earlier period or not at all.
Legend makes him the husband of Tuireann, the sister-in-law of Fionn Mac Cumhaill. Shortly before the birth of Illan’s two sons, his supernatural mistress changed Tuireann into a wolfhound, and it was in that form that she gave birth to Bran and Sgeolan, the two famous hounds of Fionn mac Cumhaill. After Illan had promised to desert Tuireann, Illan’s mistress restored her human form.
Arthurian commentators have sought to argue that he was a historical enemy of Arthur.