Muirchertach Mac Toirdelbach Mor Ua Briain, High King of Ireland

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Muirchertach Mac Toirdelbach Mor Ua Briain, High King of Ireland

Also Known As: "King of Munster"
Birthplace: Munster, Ireland
Death: March 10, 1119 (64-73)
Lismore, County Waterford, Ireland
Immediate Family:

Son of Toirdelbach Ó Briain, High King of Ireland and Derborgaill ingen Taidg O'Caellaide
Husband of Dubhchobhleig O'Brien
Father of Lafracoth ingen Muiredaig Ó Briain; Dermad Murchadh, King of Leinster and Matgamain MacMuirchertach
Brother of Tadc mac Toirdelbach Ua Briain
Half brother of Diarmaid Mactoirrdelbaig Ó Brian, King of Munster; Derbail Ó Caellaide (O Brian); Áed in Gaí Bernaig Ó Conchabhair, King of Connaught and Finn mac Dunlaing O'Caellaide

Occupation: King of Munster, King og Ireland, High King of Ireland
Managed by: Bo Garsteen
Last Updated:

About Muirchertach Mac Toirdelbach Mor Ua Briain, High King of Ireland

Muirchertach Ua Briain (c. 1050 – c. 10 March 1119), son of Toirdelbach Ua Briain and great-grandson of Brian Bóru, was King of Munster and later self declared High King of Ireland.


Major political divisions of Ireland similar to those in Muircheartach's timeMuirchertach was a son of Toirdelbach Ua Briain, a previous Dalcassian king of Ireland. 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: Tadc, Muirchertach and Diarmait. Tadc 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 province-king of Munster. He went on forays into Mide and Leinster in 1089 and took the kingship of Leinster and fought for the Viking town 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 with opposition

In 1094, Muirchertach fought the kings of Leth Cuinn and Gofraid, king of Dublin. 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. He fought for Muirchertah, aiding his defeat of 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 Blathmin Ua Briain 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 daughters 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". 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 Ua Briain:

"Muirchertach Ua Briain was an ambitious, modernizing 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 Phillip I in France ... but his actual 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 stabilizing influence in Welsh politics; he incurs trade sanctions from the king of England, and negotiates their suspension. Altogether, Muirchertach Ua 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."

a) DERBORGAILL (-after 1098). The Annals of Tigernach (Continuation) record that “Derborgaill daughter of Tadg, son of Gilla Pátraic, mother of Muirchertach Húa Bríáin king of Ireland…rested in Glendalough” in 1098[753]. m TOIRDELBACH, son of TADGH & his wife --- ([1008/09]-Cenn Coradh 14 Jul 1086). He succeeded in 1064 as King of Munster.

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