Many Irish annals state that Brian was in his 88th year when he fell in the Battle of Clontarf in 1014. If true, this would mean that he was born as early as 926 or 927. Other birth dates given in retrospect are 923 or 942.
Brian was born at Kincora, 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 Cais 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.
When their father died, the kingship of Tuadmumu passed to Brian's older brother, Mathgamain, and, when Mathgamain was killed in 976, Brian replaced him. Subsequently he became the King of the entire kingdom of Munster.
Situation of his tribe, the Dál gCais Brian belonged to the Dál gCais (or Dalcassians), a newly styled kin group of ultimately Déisi origin who occupied a territory north of the Shannon Estuary, which today would incorporate a substantial part of County Clare and then formed the core of the new kingdom of Thomond. In earlier times their ancestors had controlled some lands in today's County Limerick as well, but these had been overrun by the Uí Fidgenti from the 9th century and the invading Norse in the 10th.
The River Shannon served as an easy route by which raids could be made against the provinces of Connacht and Meath. Both Brian's father, Cennétig mac Lorcáin and his older brother Mathgamain conducted river-borne raids, in which the young Brian would undoubtedly have participated. This was probably the root of his appreciation for naval forces in his later career. Thus an important influence upon the Dalcassians was the presence of the Hiberno-Norse city of Limerick on anisthmus around which the Shannon River winds (known today as King's Island or the Island Field). The Norse had made many a raid themselves from the Shannon, and the Dalcassians likely benefited from some interaction with them, from which they would have been exposed to innovations such as superior weapons and ship design, all factors that may have contributed to their growing power.
Reign of his brother, MathgamainIn 964, Brian's older brother, Mathgamain, claimed control over the entire province of Munster by capturing the Rock of Cashel, capital of the ancient Eóganachta, the hereditary overlords or High Kings of Munster, but who in dynastic strife and with multiple assassinations had weakened themselves to the point they were now impotent. Earlier attacks from both the Uí Néill and Vikings were also factors. This situation allowed the illegitimate (from the Eóganacht perspective) but militarized Dál Cais to attempt to seize the provincial kingship. However, Mathgamain was never fully recognized and was opposed throughout his career in the 960s and 970s byMáel Muad mac Brain, a semi-outsider from the Cashel perspective but still a legitimate Eóganacht claimant from far south Munster. In addition to Máel Muad, the Norse king Ivar of Limerick was a threat and may have been attempting to establish some overlordship in the province or a region of it himself, with the Cogad Gaedel re Gallaib even asserting he actually achieved this until routed by Mathgamain in the celebrated Battle of Sulcoit in 967. This victory was not decisive however and eventually there grew up a brief alliance of sorts between Mathgamain, Máel Muad and others to drive the Norse "soldiers" or "officials" out of Munster and destroy their Limerick fortress in 972. But the two Gaelic claimants were soon back to fighting and the fortuitous capture of Mathgamain in 976 by Donnubán mac Cathail allowed him to be effortlessly dispatched or murdered by Máel Muad, who would now rule as king of Cashel for two years.
But the Dál Cais remained a powerful force and Brian quickly proved to be as fine a commander of armies as his brother. After first dispatching the already much weakened Ivar in 977, he challenged Máel Muad in 978 and defeated him in the fateful Battle of Belach Lechta, after which all the Eóganachta were no longer viable at the provincial level and Brian and the Dál Cais now enjoyed the overlordship, although not the traditional kingship of the province, which was based on lineage. Either soon before or soon after his victory over Máel Muad, Brian routed Donnubán and the remainder of the Norse army in the Battle of Cathair Cuan, there probably slaying the last of Ivar's sons and successor Aralt. He then allowed some of the Norse to remain in their settlement, but they were wealthy and now central to trade in the region, with a fleet of great value.
Cian, the son of his brother Mathgamain's sworn enemy Máel Muad, later became a loyal ally of Brian and served under him in a number of campaigns.