In the only statement of his that we know about, he describes himself as Imperator Scottorum, Emperor of the Irish. Imperator means a man who rules over many different peoples. And he saw himself as ruling equally over the Irish and the Vikings. He subjected Limerick to himself and made Limerick a dynastic capital. He subjected Cork and Waterford to himself. Dublin was next on the list.
In Dublin city hall, the legend of Brain is commemorated on the dome. In the telling of Ireland’s story, he would become an icon of native resistance, the first nationalist hero--his soldiers, holy warriors who defeated a Viking invasion. But the truth is more complex.
In 1014, after defeating the city of Waterford, Brian moved confront the Gaelic kingdom of Leinster and the Viking port of Dublin. Irish and Viking united in defense against Brian. They recruited Viking mercenaries from Britain. It’s thought Brian too had Vikings in his army. For both sides, Dublin was the glittering prize.
The battle of Clontarf is not a battle between savage Vikings and the Irish. It’s not the saving of holy Ireland from the Pagans. It is a power struggle in which Brian Boru was finally going to get Dublin, because every king wanted to control the trading cities.