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 faithful ally of Brian and served under him in a number of campaigns.