Brian Bóruma mac Cennétig (c. 941–23 April 1014) (English: Brian Boru, Middle Irish: Brian Bóruma, Irish: Brian Bóroimhe), is an Irish king who ends the domination of the High Kingship of Ireland by the Uí Néill.

Building on the achievements of his father, Cennétig mac Lorcain, and especially his elder brother, Mathgamain, Brian first makes himself King of Munster, then subjugates Leinster, making himself ruler of the south of Ireland.

He is the founder of the O'Brien dynasty.

With a population of under 500,000 people, Ireland has over 150 kings, with greater or lesser domains.

The Uí Néill king Máel Sechnaill mac Domnaill, abandoned by his northern kinsmen of the Cenél nEógain and Cenél Conaill, acknowledges Brian as High King at Athlone in 1002.

In the decade that follows, Brian campaigns against the northern Uí Néill, who refuse to accept his claims, against Leinster, where resistance is frequent, and against the Norse Gaelic Kingdom of Dublin.

Brian's hard-won authority is seriously challenged in 1013 when his ally Máel Sechnaill is attacked by the Cenél nEógain king Flaithbertach Ua Néill, with the Ulstermen as his allies.

This is followed by further attacks on Máel Sechnaill by the Dubliners under their king Sihtric Silkbeard and the Leinstermen led by Máel Mórda mac Murchada.

Brian campaigns against these enemies in 1013.

In 1014, Brian's armies confront the armies of Leinster and Dublin at Clontarf near Dublin on Good Friday.

The resulting Battle of Clontarf is a bloody affair, with Brian, his son Murchad, and Máel Mórda among those killed.

The list of the noble dead in the Annals of Ulster includes Irish kings, Norse Gaels, Scotsmen, and Scandinavians.

The immediate beneficiary of the slaughter is Máel Sechnaill, who resumes his interrupted reign.

The court of Brian's great-grandson Muirchertach Ua Briain produces the Cogadh Gaedhel re Gallaibh, a work of near hagiography.

The Norse Gaels and Scandinavians too produce works magnifying Brian, among these Njal's Saga, the Orkneyinga Saga, and the now-lost Brian's Saga.

Brian's war against Máel Mórda and Sihtric is to be inextricably connected with his complicated marital relations, in particular his marriage to Gormlaith, Máel Mórda's sister and Sihtric's mother, who had been in turn the wife of Amlaíb Cuarán, king of Dublin and York, then of Máel Sechnaill, and finally of Brian.