Ailill Ollamh (or Oilill Olum) in Irish traditional history was the son of Mug Nuadat and was a king of the southern half of Ireland. Sadb ingen Chuinn, daughter of Conn of the Hundred Battles, in her second marriage, married Ailill. He divided the kingdom between his sons Éogan, Cormac Cas, and Cian. Éogan founded the dynasty of the Eóganachta. Sadb's son Lugaid mac Con, who was Ailill's foster-son, became High King of Ireland.
The Book of Leinster contains poems ascribed to him.
The O'Sullivans are one of the number of surnames listed below as descendants of Ailill Ollamh. The An Leabhar Muimhneach (Book of Munster) has an extensive genealogy of the Eóganacht septs.
Ailill, King of Munster, discovered that the grass in his fields would not grow. Without the grass, there were no herds; without the cattle, his people would starve. Ferchess the Druid told him to go to Knockainey at Samhain Eve. When he arrived, Ailill became drowsy and fell into a half-sleep in which he had a vision of Áine, the goddess of plentiful crops and fertility. Overcome with desire, he forced himself upon her, an assault ending in Áine biting off his ear, hence the name Aulom "one-eared". By Ancient Irish law, only an "unblemished" person can rule, and by maiming him so, Áine rendered him unfit to be king. As an embodiment of sovereignty, she can both grant and remove a man's power to rule.