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.

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.

[9][8][11] 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.

Mathgamain was never fully recognized and was opposed throughout his career in the 960s and 970s by Máel 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 and Brian in the celebrated Battle of Sulcoit in 968.

This victory was not decisive 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.

[16] 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.

However, in an act of revenge for the death of his brother, Brian set aside the sacred traditions and attacked the island with his troops, slaughtered the Vikings and desecrated the church.

In any case, the event is mentioned in the Annals, and the later source the Cogadh describes Brian making a "great slaughter" of his enemies, killing both Donnubán and Aralt, and securing his position within the province.

[19] 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.

[23][citation needed] Mael Sechnaill launched a counter-offensive into Munster and in the ensuing battle he defeated the Dál gCáis, killing around six hundred men, including Brian's uncle.

The Irish annals all agree that this was a particularly fierce and bloody engagement, although claims that it lasted from morning until midnight, or that the combined Leinster-Dublin force lost 4,000 killed are open to question.

Brian made it clear that his ambitions had not been satisfied by the compromise of 997 when, in the year 1000, he led a combined Munster-Leinster-Dublin army in an attack on High King Máel Sechnaill mac Domnaill's home province of Meath.

These bridges would serve as both obstacles preventing Brian's fleet from traveling up the Shannon and a means by which the armies of the provinces of Meath and Connacht could cross over into each other's kingdoms.

Máel Sechnaill fails to rally the regional rulers who are nominally his subordinates by the time the deadline arrives, and he is forced to surrender his title to Brian.

There have been some doubts expressed about this explanation, given Brian's style of engaging in war; if he had found his opponent at a disadvantage it is most likely he would have taken full advantage of it rather than allowing his enemy the time to even the odds.

It took Brian ten years of campaigning to achieve his goal, which, considering that he could and did call on all of the military forces of the rest of Ireland, indicates how formidable the kings of Ulster were.

This led to Brian's return to Ulster yet again, this time taking hostages from the Cenél Eoghain back to his home province, finally gaining the proper submission of Flaithbertaigh.

To compound Brian's problems, Máel Mórda's Norse contingents, led by Sigurd Hlodvirsson, Earl of Orkney, and Brodir of the Isle of Man, arrived on Palm Sunday, 18 April.

According to one account, Maél Sechnaill had a change of heart and arrived late to the battle and, after the death of Brian, led the Irish army and completed the rout.

He was killed by fleeing Viking mercenaries shortly after learning from his attendant that, despite his forces having won a great victory, his son Murchadh's standard had just fallen.

[19] According to the 12th century Cogad Gáedel re Gallaib, Brian was visited in a dream on the night prior to the battle by a Pre-Christian goddess from the Irish mythology named Aibell.

Aibell had been demoted since St Patrick to a Fairy Queen ruling over the Celtic Otherworld of Thomond and was traditionally considered the guardian spirit, protectress, and the banshee of King Brian's clan and dynasty.

Believing that the dream predicted his destiny and also, it is implied, not wishing to outlive his son Murchadh, Brian refused to seek shelter from the retreating Vikings and even chose to dismiss his bodyguards at the critical moment.

[41] According to Njáls saga, on the other hand, Brodir was captured almost immediately afterwards and brutally tortured to death by Ulf the Quarrelsome, a bodyguard and possibly a relative through marriage of Brian and who was fiercely loyal to the late king.

[42] After his death, Brian's body was taken, as he had instructed moments before Brodir's arrival, for his Requiem Mass to the church upon Spittal Hill, in what is now Swords, County Dublin and then to Armagh to be buried in the Cathedral founded by St Patrick.

However, revisionist historians see it as an Irish civil war in which Brian Boru's Munster and its allies defeated Leinster and Dublin, and that there were Vikings fighting on both sides.

[47][48] In January 2018 researchers from the Universities of Coventry, Oxford and Sheffield, led by Coventry University professor Ralph Kenna, a theoretical physicist, published a paper[2] in the journal Royal Society Open Science that used network science to mathematically analyse the 12th-century Cogadh Gáedhel re Gallaibh ("The War of the Gaels with the Foreigners", i.e. the Danes and other Norsemen), that listed over 1000 relationships between about 300 characters, and concluded that the standard and popular view of the war between the Irish and Germanic Norsemen was broadly correct, but that the picture was nevertheless more complex than "a fully 'clear-cut' Irish versus Viking conflict".

[48] The revisionist theory is that the popular image of Brian—the ruler who managed to unify the regional leaders of Ireland so as to free the land from a 'Danish' (Viking) occupation—originates from the powerful influence of the Cogadh Gaedhil re Gallaibh, in which Brian takes the leading role.

His exploits in battle serve as the inspiration for Irish folk metal band Cruachan's song, Born for War (The Rise of Brian Boru).