Illann (d. 527?), son of Dúnlaing and putatively overking of Leinster, was an early representative of the Uí Dúnlainge dynasty. His father Dúnlaing (qv) son of Énnae Nia, allegedly a grandson of Bressal Bélach, is eponymous ancestor of the dynasty; his mother is said to have been Cuach daughter of Cóelbad of the Uí Bairrche. According to the genealogies, Illann had two full brothers, Ailill and Eochaid, and four half-brothers: Fergus, Nad Buidb, Dubthach Lethderg, and Móenach (or Óengus) Dilmenn.
Illann and Ailill apparently came to prominence in the late fifth century. ‘Bethu Phátraic’ locates the brothers at Naas at the time St Patrick (qv) undertook his alleged circuit of Ireland. It is claimed that he baptised them both and received Ailill's daughters Mugain and Fedelm into the religious life. According to the annals, Illann and Ailill supported the Uí Neill king Muirchertach/Mac Ercae (qv) against Óengus (qv) son of Nad-fraích king of Cashel at the battle of Cell Osnada in 490. The Latin Life of Ciarán (qv) of Saigir (Seirkieran) claims that the saint foresaw how Óengus would perish at the hands of Illann. The Leinster provincial king-list includes Illann as successor to the Dál Messin Corb ruler Fróech son of Finnchad, whose death is placed at 495.
The length of Illann's alleged reign is not given, but according to the annals he died in 527. A gloss in the king-list has him slain in the battle of Duma Aichir, but this may reflect a confused tradition; according to the Annals of Ulster, the Leinstermen defeated the Uí Néill in that battle in 473. The king-list names Illann's successor as his brother Ailill (d. 548, Ann. Tig.) and credits him with a reign of twenty years. Such claims, however, cannot be reconciled with the version of events in the annals; there are grounds for considering that Illann, Ailill, and the latter's immediate progeny merely enjoyed local prominence and did not hold supremacy in Leinster, the king-lists being later ‘amended’ in the interests of Uí Dúnlainge, which more likely came to prominence in the seventh century, in the time of Ailill's descendant Fáelán (qv) son of Colmán.