In Irish traditional history Ailill Ollamh (or Oilill Olum), son of Mug Nuadat, was a king of the southern half of Ireland. Sabia, daughter of Conn of the Hundred Battles, was his wife. He divided the kingdom between his sons Éogan, Cormac Cas, and Cian. Éogan founded the dynasty of the Eóganachta. His foster-son, Lugaid mac Con, became High King of Ireland.
The Book of Leinster contains poems ascribed to him.