The impatient MacMurrough returned to Ireland with a handful of Normans in 1167, but O'Connor and O'Rourke soon forced him to submit. Always the master of deceit, MacMurrough even paid O'Rourke one hundred ounces of gold as reparation for abducting Dervorgilla. But MacMurrough was not discouraged. He knew that help was on the way. 

The first Norman troop ships, about 600 in number, landed at Bannow Bay early in May 1169. MacMurrough and several hundred of his men promptly joined the Normans, and together they marched on Wexford. The Viking inhabitants directly confronted the invaders, expecting to find a rag-tag outfit of enthusiastic but poorly armed Irishmen. Instead they discovered a fully armed and disciplined professional army, ready for the kill. The Vikings were driven back into Wexford, and next day the town was forced to surrender.

Strongbow himself now set sail for Ireland. His advance guard, ten knights and seventy archers, was led by a magnificent young soldier-warrior from the FitzGerald family, Raymond Carew, commonly called 'le Gros' ('the Fat'). Le Gros landed north of Waterford and quickly built earthen ramparts which remain even today. Almost immediately, an opposition army -- several thousand Vikings and Gaelic-Irish from Waterford and the surrounding areas -- attacked le Gros and his contingent of eighty Norman and Fleming soldiers.;

Incredibly, le Gros and his vastly outnumbered troops prevailed. Behind the ramparts, le Gros had concealed a herd of cattle, which he suddenly stampeded into the oncoming troops, trampling the front rank of the attackers. In all the confusion, le Gros and his force routed the remaining natives, seventy of whom were captured alive. As a message to Waterford, the prisoners' limbs were broken, their heads severed, and their bodies thrown over the cliffs.

Now Strongbow and his army of about two hundred knights and a thousand other troops joined le Gros, and two days later, they attacked Waterford. Twice the Normans were beaten off, but eventually le Gros breached the walls at a weak point, and captured Waterford.

Now MacMurrough's daughter, Aoife, came to Waterford, where she married Strongbow. A celebrated fresco in the British House of Commons depicts the wedding ceremony occurring at the close of the battle, against a background of burning buildings and dead Irishmen. The fresco is a classic example of artistic license, since Aoife did not arrive until several days after the battle. But the artist captured brilliantly the essential elements of the pact which MacMurrough had made with Strongbow two years earlier in Wales.

Strongbow and MacMurrough now set their sights on taking Dublin, which was a semi-independent Viking kingdom. With the wily MacMurrough leading the way, the Normans evaded an ambush laid by O'Connor and O'Rourke and arrived unscathed at the city walls. The Vikings were inclined to surrender, but while negotiations were still ongoing, le Gros and Milo de Cogan led their troops through a breach in the city walls and routed the city's ineffectual defenders. Asgall, the Viking-Irish King of Dublin, managed to escape with some of his Viking followers. As they sailed away, Asgall vowed to return and retake Dublin.