MacMurrough became deathly ill in April 1171, and while Strongbow was visiting him, Asgall, who had been forced to flee only nine months earlier, made good on his vow to return. He brought with him a fleet of ships carrying about a thousand Vikings, who mounted a fierce assault on Strongbow's Dublin. But even without Strongbow to lead them, the Normans prevailed. Asgall, who was taken prisoner, was tried, convicted and beheaded in the hall of what formerly had been his own palace in Dublin.

Upon MacMurrough's death, Strongbow returned to Dublin, only to be confronted by a revolt of the Leinster tribes, who were challenging Strongbow's right to succeed MacMurrough as King of Leinster. The revolt nominally was led by Murtaugh MacMurrough, Dermot MacMurrough's nephew and heir, who claimed that succession should be determined by Irish (Brehon) law, not his uncle's agreement with Strongbow.

Significantly, Murtaugh had military support from the Ard Ri, Rory O'Connor, along with widespread support from the other Gaelic lords. The Gaelic plan was to lay siege to Dublin and to starve the Normans into surrender. But after two months, complacency set in, which gave the Normans their opportunity.

Strongbow, le Gros, and Milo de Cogan, each with a contingent of 200 men, snuck out of the city, circled around behind O'Connor, and mounted a surprise attack. The daring initiative succeeded beyond all expectations, decimating O'Connor's army and terminating the siege. The victory established once and for all the Norman's military supremacy over both Viking and Gaelic Irish.

Nominally, Strongbow and his Norman troops were acting on behalf of Henry II, but Henry suspected -- with good reason -- that Strongbow planned to establish an independent kingdom in Ireland. But Henry wanted Ireland to become part of his Angevin empire, with the Normans serving as feudal underlords. To squelch any contrary plans Strongbow might have, Henry promptly traveled to Ireland, bringing with him 4,000 well armed troops as a show of force. The visit was an enormous success. Without a drop of blood being shed, Strongbow and the other Norman warriors capitulated and paid homage to Henry, as did virtually everyone else of importance, the Normans, the Irish, the Vikings, even the bishops. Ireland was now part of Henry's Angevin empire.