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.