At a lecture in the RIA, 4/3/2014, Sean Duffy, Associate Professor of Medieval History at TCD and author of a new book on the subject, took issue with the idea of the Battle of Clontarf as a Munster versus Leinster conflict. He says it was a battle for the sovereignty of Ireland. In the Book of Armagh, Brian is referred to as ‘Imperator Scotorum’ (‘Emperor of the Gaels’), and indeed he appears to have claimed kingship over the Gaels of Britain as well as Ireland. In the Irish Annals there is reference to the ‘Slaughter of the Foreigners of the Western World’. The clearest account of the Battle of Clontarf is in the Annals of Inisfallen.
It was preceded in 980 by the battle of Tara where Olaf made a grab for the high kingship of Ireland. In 999 Brian Boru confronted Olaf’s son, Silken Silkenbeard, King of Dublin, at the Battle of Glen Mamu. (Nobody knows where this place is.)
Duffy situates the Battle of Clontarf against a background of events in England. There were Danish attacks on England in 1006. In July 1013 Sven Forkbeard attacked Aethelred ‘the Unready’, who fled to Norway. Aethelred returned and confronted Knut (Canute), the son of Forkbeard. Knut fled England in April 1014. The victory of Aethelred was very probably known at the time of the Battle of Clontarf, and it must have encouraged the Irish.