Brian Boru is the most famous Irish person before the modern era, whose death at the Battle of Clontarf in 1014 is one of the few events in the whole of Ireland's medieval history to retain a place in the popular imagination. Once, we were told that Brian, the great Christian king, gave his life in a battle on Good Friday against pagan Viking enemies whose defeat banished them from Ireland forever. More recent interpretations of the Battle of Clontarf have played down the role of the Vikings and portrayed it as merely the final act in a rebellion against Brian, the king of Munster, by his enemies in Leinster and Dublin. This book proposes a far-reaching reassessment of Brian Boru and Clontarf. By examining Brian's family history and tracing his career from its earliest days, it uncovers the origins of Brian's greatness and explains precisely how he changed Irish political life forever. Brian Boru and the Battle of Clontarf offers a new interpretation of the role of the Vikings in Irish affairs and explains how Brian emerged from obscurity to attain the high-kingship of Ireland because of his exploitation of the Viking presence. And it concludes that Clontarf was deemed a triumph, despite Brian's death, because of what he averted – a major new Viking offensive in Ireland – on that fateful day.