Fíngen mac Áedo Duib in the context of Faílbe Flann mac Áedo Duib

Eóganacht Chaisil were a branch of the Eóganachta, the ruling dynasty of Munster between the 5th and 10th centuries. They took their name from Cashel (County Tipperary) 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. The McGillycuddy are a sept of the O'Sullivan's. 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.

O'Sullivan (Irish: Ó Súilleabháin, Súileabhánach) is an Irish Gaelic clan based most prominently in what is today County Cork and County Kerry. According to traditional genealogy, the O’Sullivans were descended from the ancient Eóganacht Chaisil sept of Cenél Fíngin, the founder of the clan who was placed in the 9th century, eight generations removed from Fíngen mac Áedo Duib, king of Cashel or Munster from 601 to 618.Later, they became the chief princes underneath their close kinsmen, the MacCarthy dynasty, in the small but powerful Kingdom of Desmond, successor of Cashel/Munster.The last independent ruler of the clan was Donal Cam O'Sullivan Beare, who was defeated in the Nine Years' War of 1594–1603.