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-GaelicKingdom of Dublin. 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 Dubliners under their king Sihtric Silkbeard and the Leinstermen led by Máel Mórda mac Murchada. Brian campaigned against these enemies in 1013. In 1014, Brian's armies confronted the armies of Leinster and Dublin at Clontarf near Dublin on Good Friday.
The resulting Battle of Clontarf was 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 was Máel Sechnaill who resumed his interrupted reign. The Norse-Gaels and Scandinavians also produced works mentioning Brian, among theseNjal's Saga, the Orkneyinga Saga, and the now-lost Brian's Saga. Brian's war against Máel Mórda and Sihtric was 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.