Dál Birn in the context of Kingdom of Ossory

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.

Osraige (Old Irish) or Osraighe (Classical Irish), Osraí (Modern Irish), anglicized as Ossory, was a medieval Irish kingdom comprising what is now County Kilkenny, western County Laois and parts of eastern County Tipperary, corresponding to the Diocese of Ossory. The home of the Osraige people, it existed from around the first century until the Norman invasion of Ireland in the 12th century. It was ruled by the Dál Birn dynasty, whose medieval descendants assumed the surname Mac Giolla Phádraig.

According to tradition, Osraige was founded by Óengus Osrithe in the 1st century and was originally within the province of Leinster. In the 5th century, the Corcu Loígde of Munster displaced the Dál Birn and brought Osraige under Munster's direct control. The Dál Birn returned to power in the 7th century, though Osraige remained nominally part of Munster until 859, when it achieved formal independence under the powerful king Cerball mac Dúnlainge. Osraige's rulers remained major players in Irish politics for the next three centuries, though they never vied for the High Kingship. In the early 12th century, dynastic infighting fragmented the kingdom, and it was re-adjoined to Leinster. The Normans under Strongbow invaded Ireland beginning in 1169, and most of Osraige collapsed under pressure from Norman leader William Marshal. The northern part of the kingdom, eventually known as Upper Ossory, survived intact under the hereditary lordship until the reign of King Henry VIII of England, when it was formally incorporated as a barony of the same name.