The husband of Medb in the Táin Bó Cuailnge [Cattle Raid of Cooley]. Although he is often described as being ‘of Connacht’, like his wife, the text of the Táin describes him as coming from Leinster, of the family of Russ Ruad. His patronymic distinguishes him from other Ailills, but it is not often used in the text of narratives. He appeared somewhat henpecked in the pillow-talk opening of the Táin but Medb praised him for many qualities, including bravery. Nevertheless, Ailill persisted in demanding that Medb should not have more possessions than he did. Their daughters are Finnabair and Faife.
In another story, Táin Bó Fraich [Cattle Raid of Fraich], Ailill is a stern father and wily king who opposes the marriage of his daughter Finnabair with Fráech to the point of nearly killing the younger man, relenting only when Fráech kills a dragon and recovers a ring swallowed by a salmon.
He assists Angus Óg to besiege Ethal Anbúail, with whose daughter Angus Óg has fallen in love. He sends Nera on his quest in Echtra Nerai [The Adventure of Nera]. Later, Ailill is slain by Conall Cearnach at Medb's urging when she finds her husband unfaithful.