Eóganacht Chaisil were a branch of the Eóganachta, the ruling dynasty of Munster during the 5th-10th centuries. They took their name from Cashel (Tipperary County) which was the capital of the early Catholic kingdom of Munster. They were descended from Óengus mac Nad Froích (died 489), the first Christian King of Munster, through his son Feidlimid mac Óengusa.
In the seventh century, they split into two main clans. Cenél Fíngin descended from Fíngen mac Áedo Duib (d. 618) and became the O'Sullivans and MacGillycuddys. A descendant of Fíngen was Feidlimid mac Cremthanin (d. 847). Clann Faílbe descended from Faílbe Flann mac Áedo Duib (d. 639) and became the MacCarthy dynasty, rulers of the Kingdom of Desmond following their displacement by the Normans. The O'Callaghans belong to the same line as the MacCarthys, while the MacAuliffes are a sept of the MacCarthys.
The Eóganacht Chaisil were considered part of the inner circle of Eoganachta dynasties which included the Eóganacht Glendamnach and Eóganacht Áine branches. These three branches rotated the kingship of Munster in the 7th and much of the 8th centuries. The Chaisil branch provided most of the kings in the 9th and 10th centuries in Munster. Kings of Cashel and Munster from the Eóganacht Chaisil were: