Diarmait Ua Briain was a 11th century Irish king who ruled Munster

Munster

Munster is one of the Provinces of Ireland situated in the south of Ireland. In Ancient Ireland, it was one of the fifths ruled by a "king of over-kings" . Following the Norman invasion of Ireland, the ancient kingdoms were shired into a number of counties for administrative and judicial purposes...

 from 1114 to 1118.

One of three sons of Toirdelbach Ua Briain, he was banished from Munster by his brother Muirchertach Ua Briain shortly after the death of their father in 1086. He lived in exile for several years while his brother ruled as King of Munster

Kings of Munster

The name Munster is derived from the Gaelic God, Muman. The province of Munster was once divided into six regions: Tuadh Mhuman , Des Mhuman , Aur/Ur Mumhan , Iar mumhan or Iarmuman , Ernaibh Muman , and Deisi Muman...

. He is believed to have been the commander of the Irish fleet that supported Rhys ap Tewdwr

Rhys ap Tewdwr

Rhys ap Tewdwr was a Prince of Deheubarth in south-west Wales and member of the Dinefwr dynasty, a branch descended from Rhodri the Great...

 at the Battle of Mynydd Carn

Battle of Mynydd Carn

The Battle of Mynydd Carn took place in 1081, as part of a dynastic struggle for control of the Welsh kingdoms of Gwynedd and Deheubarth. The result of the battle had a radical effect on the history of Wales....

.

In 1093, he reconciled with Muirchertach after recognizing his claim and was installed as dux of Waterford

Waterford

Waterford is a city in the South-East Region of Ireland. It is the oldest city in the country and fifth largest by population. Waterford City Council is the local government authority for the city and its immediate hinterland...

. He eventually succeeded his brother as acting King of Munster when his brother became seriously ill in 1114. After taking the throne, he banished Muircheartach and soon was involved in a four-year struggle for control of Munster. In 1116, he attacked Muirchertach and Brian Ua Briain "in violation of a mutual oath on the relics of Ireland" besieging Limerick

Limerick

Limerick is the third largest city in the Republic of Ireland, and the principal city of County Limerick and Ireland's Mid-West Region. It is the fifth most populous city in all of Ireland. When taking the extra-municipal suburbs into account, Limerick is the third largest conurbation in the...

. This resulted in Muirchertach making peace with Diarmait, however he was eventually captured by Muircheartach.

He later died in Cork

Cork (city)

Cork is the second largest city in the Republic of Ireland and the island of Ireland's third most populous city. It is the principal city and administrative centre of County Cork and the largest city in the province of Munster. Cork has a population of 119,418, while the addition of the suburban...

 in 1118 and in the same year, Toirdelbach Ua Conchobair invaded Munster and partitioned the land between the sons of Diarmait Ua Briain and Tadg Mac Carthaig, Conchobar Ua Briain of Thomond

Thomond

Thomond The region of Ireland associated with the name Thomond is County Clare, County Limerick and north County Tipperary; effectively most of north Munster. The name is used by a variety of establishments and organisations located in , or associated with the region...

 and Toirdelbach Mac Carthaig of Desmond

Desmond

Desmond may refer to:*Desmond , a common given name and surname* Kingdom of Desmond, medieval Irish kingdom* Earl of Desmond, Irish aristocratic title* Desmond Rebellions, Irish rebellions during the 16th century led by the Earl of Desmond...

respectively.