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'Sullivan's are descendants of Ailill Ollamh.