Dermot, the King of Leinster in eastern Ireland, was defeated in battle by Tighernan O Ruairc, another ruler in the region. Dermot was exiled and went to Normandy and the court of Henry II of England to ask for assistance in retaking his lands. Henry gave Dermot permission to find a willing army from either England or Wales and so Dermot approached Richard de Clare, the Earl of Pembroke who agreed to help in return for several demands. These demands were Dermot's daughter's hand in marriage and to rule Leinster after Dermot died.

Some the men promised to assist Dermot in Ireland landed at Bannow with the sons of Nesta. The poorly protected Irish warriors were no match for the armoured knights and archers that arrived from Wales. By the end of the year Leinster fell and Dermot once again became the King. After his victory Dermot was not satisfied with just ruling Leinster but wanted to become High King of Ireland. To fulfil his ambitions he had to wait for the arrival of Strongbow, the Earl of Pembroke, for that to be a possibility.

Raymond de Gros arrived in Ireland in support of Dermot a few months ahead of Strongbow. He landed at Waterford and constructed a fort in which he and his men prepared for the arrival of the Earl.

Richard de Clare, the Earl of Pembroke, invaded Ireland with 200 knights in armour and up to a 1,000 foot-soldiers capturing Waterford easily and then Dublin on behalf of Dermot, the King of Leinster.

After letting the population of Dublin leave, the Earl of Pembroke and his men spent the Winter in the city.

The agreement between the Earl of Pembroke and Dermot, the King of Leinster, was that if the Earl helped the King retake Leinster the Earl would become the ruler of Leinster when Dermot died. When the king died in the spring of 1171 the agreement fell apart due to the lack of support from Dermot's tribesmen. The tribesmen elected a nephew of Dermot's as the new king instead.

King Henry II had become concerned that Strongbow was building a strong base in Ireland, strong enough to become an independent state separate from England. Henry planned to take an army to Ireland to deal with Strongbow but Strongbow travelled to England and in September met Henry and promised his loyalty to the King.

King Henry II travelled to Ireland to receive the homage of the Irish leaders. Traders were invited to Dublin where a colony of English was set up. Henry stayed in Ireland over the winter months returning to England in the spring of 1172.

Richard 'Strongbow' fitzGilbert de Clare, Earl of Pembroke died in Ireland.