In Irish mythology Ailill was king of Connacht and husband of Medb during the events of the Ulster Cycle.

He was the owner of the phenomenally fertile White-horned bull of Connacht, called Finnbhennach. Although the bull was born into Medb's herd, the legend says that the animal thought that belonging to a woman was lower in dignity than belonging to a man, and transferred itself into Ailill's ownership. When it was discovered that this bull was the difference in wealth between them, Medb started the Táin Bó Cúailnge (Cattle Raid of Cooley) to steal the equally potent Donn Cuailnge from Ulster and restore equality with her husband.

He had to satisfy Medb's critieria that her husband must be without fear, meanness and jealousy - especially the latter as Medb had many lovers. Despite this he contrived the death of Fergus mac Róich over the latter's affair with Medb, and was himself killed by Conall Cernach, at Medb's prompting, in revenge for Fergus.