Brian Boru (Middle Irish: Brian Bóruma mac Cennétig; modern Irish: Brian Bóramha; c. 941 – 23 April 1014) was the High King of Ireland from 1002 to 1014. He ended the domination of the High Kingship of Ireland by the Uí Néill, and is likely responsible for ending Viking invasions of Ireland. Brian Boru is mentioned in the Annals of Inisfallen and in Chronicon Scotorum as "Brian mac Cennétig" (Brian, son of Cennétig). The name Brian of Bóruma or Brian Boru was given to him posthumously. Brian built on the achievements of his father, Cennétig mac Lorcain, and especially his elder brother, Mathgamain. Brian first made himself king of Munster, then subjugated Leinster, eventually becoming High King of Ireland. He was the founder of the O'Brien dynasty, and is widely regarded as one of the most successful and unifying monarchs in medieval Ireland.

With a population of under 500,000 people, Ireland had 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, acknowledged Brian as High King at Athlone in 1002. In the decade that followed, Brian campaigned against the northern Uí Néill, who refused to accept his claims; against Leinster, where resistance was frequent; and against the Norse-Gaelic Kingdom of Dublin. Brian was described in the Annals of Ulster as ardrí Gaidhel Erenn & Gall & Bretan, August iartair tuaiscirt Eorpa uile (High King of the Gaels of Ireland and the Norse foreigners and the Britons, Augustus of all north-western Europe), the only Irish king to receive that distinction in the annals.

Brian's hard-won authority was seriously challenged in 1013 when his ally Máel Sechnaill was attacked by the Cenél nEógain king Flaithbertach Ua Néill, with the Ulstermen as his allies. This was followed by further attacks on Máel Sechnaill by the Germanic Norsemen of Dublin under their Norse king Sigtrygg Silkbeard and the Leinstermen led by Máel Mórda mac Murchada. Brian campaigned against these enemies in 1013. In 1014, his armies confronted the armies of Leinster and Dublin. In the resulting Battle of Clontarf Brian was killed; nonetheless, his army was victorious against the Leinstermen and Norsemen. The battle is widely lauded as a pivotal moment in Irish history, and is well known in popular memory.

Brian was well regarded by contemporary chroniclers. The Norse-Gaels and Scandinavians also produced works mentioning Brian, including Njal's Saga, the Orkneyinga Saga, and the now-lost Brian's Saga. Brian's war against Máel Mórda and Sigtrygg was to be inextricably connected with his complicated marital relations, in particular his marriage to Gormlaith, Máel Mórda's sister and Sigtrygg'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 himself.

Brian Boru was one of the twelve sons of Cennétig mac Lorcáin (d. 951), who was King of Dál gCais and king of Tuadmumu (Thomond), modern County Clare, then a sub-kingdom in the north of Munster. Cennétig was described as rígdamna Caisil, meaning that he was either heir or candidate ("king material") to the kingship of Cashel or Munster, although this might be a later interpolation. Brian's mother was Bé Binn inion Urchadh, daughter of Urchadh mac Murchadh (d. 945), king of Maigh Seóla in west Connacht. That they belonged to the Uí Briúin Seóla may explain why he received the name Brian, which was rare among the Dál gCais.

Brian's family were descended from the Ui Tairdelbach branch of the Dal gCais (or Deis Tuisceart). This branch had recently taken power from the Ui Óengusso branch which had traditionally supplied the kings of the Dal gCais (also known as the Deisi Tuaiscirt). The Uí Toirdhealbhach had extended their influence over Thomond and in 925 the annals note that Ánrothán son of Máel Gorm assumed the kingship of Corco Mruad. In 934 Reabachán mac Mothla died and was named as "King of the Dál gCais" in the Annals (the earliest usage of this term). The same year of his death, his son Dub Gilla was killed by Brian's uncle Conghalach and Brian's grandfather Lorcáin is said to have succeeded to the kingship. He enjoyed a short reign before being succeeded by his own son Cinnéidigh who was the first King of Dál gCais to lead an army outside his own territory and lead an expedition as far north as Athlone. By his death in 951, he had been acknowledged as "King of Tuadmumu". His brother, Mathgamain, built on these achievements and was the first to capture Cashel and become King of Munster.

Brian was born at Kincora, his father's residence or fort in Killaloe, a town in the region of Tuadmumu. Brian's posthumous cognomen "Bóruma" (anglicised as Boru) may have referred to "Béal Bóruma", a fort north of Killaloe, where the Dál gCais held sway. Another explanation, though possibly a late (re-)interpretation, is that the nickname represented Old Irish bóruma "of the cattle tribute", referring to his capacity as a powerful overlord.

As the youngest of twelve brothers, Brian was highly unlikely to be heir to his father's throne. When he was young, he was sent to a monastery to study Latin and the history of Ireland. He was taught at the monastery on the island of Innisfallen, near modern Killarney in County Kerry. His teacher or mentor was the noted monk, Maelsuthain Ua Cearbhaill, an Eóganacht chief who had become the head of the monastery, noted for starting the Annals of Inisfallen. Ua Cearbhaill would later become Brian's anmchara (advisor). However, when Brian was ten years old, he received news that his father had been killed in battle with the Vikings of Limerick, and he was brought home.