Whereas some modern historians see the Leinstermen as Brian’s principal opponents at Clontarf, medieval sources are in no doubt that the enemy were the Norse of Dublin. The Annals of Inisfallen see Clontarf as the culmination of a ‘great war between Brian and the Foreigners of Dublin’. The Leinstermen were of course present but, as the Annals of Ulster put it, they were ‘killed on the side of the troops of the Foreigners’. In other words, it is not the case that Clontarf was primarily the climax of Leinster’s rebellion against Brian Boru. Brian marched on Dublin in the autumn of 1013 and laid Dublin under a siege, and marched on it again in the spring of 1014 because Dublin was the focus of resistance to him.
Yes, the Leinstermen threw in their lot with the Dubliners, but there is no evidence that the Leinstermen were dominant at this juncture. On the contrary, the Uí Dúnlainge over-kingship of Leinster had dwindled away by 1014 and the real opposition to Brian was led therefore by King Sitric of Dublin. Sitric was the one who recruited the Leinstermen to his banner and he also enlisted substantial external support, led primarily by Jarl (Earl) Sigurd of Orkney and by a fleet-commander called Bró∂ir/Bródar who came from the Isle of Man or perhaps northern England.