Olioll Olum

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 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.

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