Muirchertach Ua Briain

Muirchertach Ua Briain
King of Munster
Died 1119
Issue Mathgamain, Domnall, Bjaðmunjo
House Uí Briain
Father Toirdelbach Ua Briain
Mother Dearbhforgaill ingen Tadhg Mac Giolla Pádraig

Muircheartach Ua Briain (old spelling: Muirchertach Ua Briain) (also known as Murtough O'Brien) (c. 1050 – c. 10 March 1119), son of Toirdelbach Ua Briain and great-grandson of Brian Bóruma, was King of Munster and later self-declared High King of Ireland.


Major political divisions of Ireland similar to those in Muircheartach's time

Muirchertach Ó Briain was a son of Toirdelbach Ua Briain, the previous Dalcassian king Munster and de facto high king of Ireland. His mother was Derbforgaill, daughter of Tadhg Mac Giolla Pádraig of Osraige, who also bore Muirchertach's brother Tadhg.[1] In 1086 his father died and the province of Munster from which he had claimed kingship of Ireland had been split between his three sons: Tadhg, Muirchertach and Diarmait. Tadhg died soon after, and Muirchertach banished Diarmait from Munster, claiming its kingship for himself.

King of Munster

Between 1086 and 1101, Muirchertach consolidated and strengthened his position as King of Munster. He went on forays into Mide and Leinster in 1089 and took the kingship of Leinster and fought for the Viking Kingdom of Dublin. In 1093, he accepted the submission of Domnall mac Flainn Ua Maíl Shechnaill, the Uí Néill king of Tara, and also made peace with his brother Diarmait at Cashel.

King of Ireland

In 1094, Muirchertach fought the kings of Leth Cuinn and Gofraid Crobán, King of Dublin and the Isles. He went with his army to Dublin and banished Gofraid, and brought about the killing of Domnall Ua Maíl Shechnaill. He asserted supremacy over the Uí Néill kingdom of Mide.

In 1101 he declared himself High King and travelled the island provinces. It was in this year that he gave the fortress at on the rock of Cashel as a gift to the Church.

Alliance with Arnulf de Montgomery

In an effort to gain military support against Henry I, Arnulf de Montgomery sent his steward, Gerald of Windsor, to Ireland to negotiate terms with Muirchertach. According to a Welsh chronicle (Sean Duffy, p. 45, 1997), Arnulf "though to make peace with the Irish and to obtain help from them. And he sent messengers to Ireland, that is Gerald the Steward (Gerald of Winsor) and many others, to ask for the daughter of King Murtart for his wife. And that he easily obtained; and the messengers came joyfully to their land. And Murtart sent his daughter and many armed ships along with her to his aid. And when the earls had exalted themselves with pride because of those events, they refused to accept any peace from the king."

De Montgomery and his brother Robert, were however defeated by Henry and fled to Ireland. The Montgomery brothers fought under Muirchertach during his campaign with Magnus Barelegs, but when de Montgomery attempted to seize the kingship for himself, Muirchertach "took his daughter away from Arnulf and gave the wanton girl in an unlawful marriage to one of his cousins. He resolved to kill Arnulf himself as a reward for his alliance, but the latter ... fled to his own people and lived for twenty years afterwards with no fixed abode." (Sean Duffy, 1997, p. 46).

Magnus Barelegs

In 1102, Muirchertach cemented an alliance with Magnus Barefoot, king of Norway by marrying his daughter Bjaðmunjo to Magnus's son, Sigurd I Magnusson. Muirchertach now took part in a campaign with Magnus to assert control over Ulster, successfully defeating opposing Irish forces. After a year of campaigning, as his army was readying to depart back to Norway, King Magnus was ambushed and killed by an Irish army in Ulster. With Magnus's death, Muirchertach's daughter's marriage was disavowed by the Norwegians, weakening Muirchertachs proclaimed position as High King.

In 1114 the king became sick to the point where "he became a living skeleton".[2] In response to the king's misfortune, his brother Diarmait took control of the kingship of Munster and banished Muirchertach. The following year Muirchertach regained his strength and undertook a campaign to regain control of Munster and successfully captured Diarmait. Only later did the king regain control of Munster.

In 1119, Muirchertach Ua Briain died.


Anthony Condon (1979, p. 398) remarked of Ó Briain:

Muirchertach Ó Briain was an ambitious, modernising and outward-looking king whose goal was to make himself king of Ireland as much as William Rufus and Henry I were kings of England; in reality his position was, perhaps, more analogous to that of Philip I in France ... but his actually authority in Ireland, especially at the height of his power in the first years of the twelfth century, greatly exceeded that of Phillip in France. ... Ua Briain ... pursued a vigorous foreign policy which was to carry his activities beyond his own shores.

In the latter regard, Condon (1979, p. 415) views

Ua Briain's activities in the Irish Sea area [as] a mixture of old and new, of pragmstisim and idealism ... But they are invested with a modern purpose. Ua Briain makes one marriage with the king of Norway, and another with one of the most powerful non-royal families in Europe; he treats with the king of Scotland; his aid to the Welsh princes acts as a stabilising influence in Welsh politics; he incurs trade sanctions from the king of England, and negotiates their suspension. Altogether, Muirchertach Ó Briain lifted his head above the domestic power struggle and sought to involve Ireland in the international politics of Europe, so that some sixty years later, these activities were still well remembered, and are reflected in the vitae of St. Flannan of Killaloe.


  1. Carrigan; History and Antiquities of the Diocese of Ossory, vol. 1, pg 51.
  2. Annals of the Four Masters, UCC Corpus of Electronic Texts (CELT) version, Part 22, accessed 21 April 2008


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