Eóganacht Chaisil in the context of McGillycuddy of the Reeks

The Kingdom of Munster (Irish: Ríocht Mhumhan) was a kingdom of Gaelic Ireland which existed in the south-west of the island from at least the 1st century BC until 1118. According to traditional Irish history found in the Annals of the Four Masters, the kingdom originated as the territory of the Clanna Dedad (sometimes known as the Dáirine), an Érainn tribe of Irish Gaels. Some of the early kings were prominent in the Red Branch Cycle such as Cú Roí and Conaire Mór. For a few centuries they were competitors for the High Kingship of Ireland, but ultimately lost out to the Connachta, descendants of Conn Cétchathach. The kingdom had different borders and internal divisions at different times during its history.

Major changes reshaped Munster in the 6th century, as the Corcu Loígde (ancestors of the Ó hEidirsceoil) fell from power. Osraige which had been brought under the control of Munster for two centuries was retaken by the Dál Birn (ancestors of the Mac Giolla Phádraig). Various subordinate groups, such as the Múscraige, switched their alliance and helped to bring the Eóganachta to power in Munster. For the next three centuries, various subgroups such as the Eóganacht Chaisil (ancestors of the Ó Súilleabháin and Mac Cárthaigh) and Eóganacht Glendamnach (ancestors of the Ó Caoimh) competed for control of Munster. Celtic Christian civilisation developed at this time and the Rock of Cashel became a seat of power. Two kings, Faílbe Flann mac Áedo Duib and Cathal mac Finguine, were able to raise Munster to the premier Irish kingdom for a time.

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.

Eóganacht Glendamnach were a branch of the Eóganachta, the ruling dynasty of Munster during the 5th-10th centuries. They took their name from Glendamnach (Glanworth, County Cork). They were descended from Óengus mac Nad Froích (died 489), the first Christian King of Munster through his son Eochaid mac Óengusa (died 522) and grandson Crimthann Srem mac Echado (died c. 542). Kings of Cashel and Munster from the Eóganacht Glendamnach were:

• Coirpre Cromm mac Crimthainn, d. 577
• Cathal mac Áedo, d. 627
• Cathal Cú-cen-máthair, d. 665
• Finguine mac Cathail, d. 696
• Ailill mac Cathail, d. 701
• Cathal mac Finguine, d. 742
• Artrí mac Cathail, d. 821

King lists for the 6th century give a virtual monopoly to the Glendamnach branch. Those found in the Laud Synchronisms were probably written for the benefit of Eóganacht Glendamnach; they may have been written at Cloyne in the mid 8th century, a monastery favourable to the Glendamnach branch. The Eóganacht Glendanmach were considered part of the 'inner circle' of Eóganachta dynasties which included the Eóganacht Chaisil and Eóganacht Áine. These three branches rotated the kingship of Munster in the 7th and much of the 8th centuries. This dynasty reached its apex in the eighth century, under Cathal mac Finguine. The last king from this branch, Artrí mac Cathail, died in 821. Their later ruling sept were the Ó Caoimh (O'Keeffes).

Faílbe Flann mac Áedo Duib (died 639) was a King of Munster from the Eóganacht Chaisil branch of the Eoganachta. He succeeded Cathal mac Áedo Flaind Chathrach of the Glendamnach branch in 628. He was the younger brother of a previous king Fíngen mac Áedo Duib (d. 618). His sobriquet Flann meant "blood-red".