The first part of this page (Brian Boru) has been re-written and now appears here.

King Henry 2nd of England was interested by the case of MacMurrough, former King of Leinster, because he had been given permission by the Pope in 1155 to go to Ireland to reform the Church there, but had not been sure how to go about it. He decided to take the opportunity to go to Ireland, but to do so under the pretence of helping MacMurrough to win back his kingdom. He pledged his full cooperation to the unsuspecting MacMurrough. MacMurrough was instructed to raise a Norman army in Wales, by contacting the Welsh Earl of Pembroke ("Strongbow"), which he did. MacMurrough gave his daughter, Aoife, to Strongbow and promised to make him King of Leinster when he died. In return Strongbow sent MacMurrough back to Ireland in 1167 while he raised an army himself.

Two years later the Norman army was ready. Strongbow sent his men ahead of him and they landed at the Viking settlement of Weis-fjord [Wexford] on the south-east tip of Ireland and, eagerly joined by MacMurrough, they defeated the Vikings. As a reward, MacMurrough let the Normans keep Wexford. When the High King O'Connor saw the power of MacMurrough's Norman ally, he relented and told MacMurrough that he didn't have to recognise O'Connor's High King-ship and could safely remain King of Leinster. MacMurrough and O'Connor then signed a peace treaty. But MacMurrough was merely biding his time.

On 23 August, 1170 Strongbow himself landed at the Viking port of Vadre-fjord [Waterford] on the south coast and quickly defeated the occupants. Encouraged by two successive gains, the Normans decided to go further and attack the Viking city-state of Dubh Linn [Dublin] itself. Fearful of the power of the Norman army, the Viking King of Dubh Linn formed an alliance with High King O'Connor and the 2 men's armies waited near the city. However, the Normans managed to slip past O'Connor's army and attacked Dubh Linn behind his line. Instead of going to his ally's aid, however, O'Connor abandoned the Vikings and returned to Connaught. The King of Dubh Linn tried desperately to mediate with the Normans, but failed and the Normans captured the city causing major damage and killing many people. The Vikings fled to the nearby Isle of Man where they built up a new army, filled with a desire for revenge. Returning in 1171, and again aided by O'Connor, the Vikings attacked Dubh Linn but failed to recapture it.

Upon MacMurrough's death in 1171, Strongbow duly became King of Leinster. This was the first British-born person to become a ruler of part of Ireland. However King Henry 2nd became scared that King Strongbow would try to cut his ties with Britain in order to rule his new Irish kingdom as an independent country, so he decided to go over himself to establish his authority over Strongbow's kingdom. He landed at Waterford in October 1171 and took control without a fight. He marched up Ireland, receiving frightened pledges of loyalty from every Irish chief and new Norman Lord he passed. (A Norman Lord was an Irish chief who had pledged loyalty to the Normans, or else a Norman settler) Only King O'Connor of Connaught and King O'Neill of Ulster refused. Henry took over Dublin, Waterford and Wexford as Norman crown property, but let Strongbow remain King of Leinster so long as he pledged loyalty to Henry.

After gaining control of Leinster and Munster, Henry got around to fulfilling his pledge to the Pope. At the Synod of Cashel in 1172, Henry made the Irish church agree that their church was in need of reformation. The Pope also ordered the Irish to accept King Henry as their ruler and appointed an Irishman, Laurence O'Toole as Papal representative to Ireland. Henry and O'Toole did not agree on the level of the monarch's involvement in the Irish church and fought to prevent Henry gaining too much, but died in the attempt. (O'Toole is now the Patron Saint of Dublin.)

In 1175, as the Normans set about fortifying their newly captured cities and founded more, O'Connor realised he would never defeat them and negotiated and signed the Treaty of Windsor recognising Henry as his overlord. This left him with his Kingdom intact and no threat of attack. Norman Lords began acquiring land from the Irish chiefs by various means and soon the island began to get interspersed with Irish Lords and Norman Lords of varying degrees of loyalty to the King.

In 1177, Henry's son, John, was made Lord of Ireland, and Henry sent him to Ireland. John was arrogant and unpleasant, made fun of the Irish, and gave large chunks of Irish land to his friends, including Tipperary to his butler, and Kildare, Cork and some of Connaught to other Norman Lords. Upon his father's death in 1199, John became King and arrived in Ireland for a second time in 1210. He drove some Norman Lords out of Meath and southern Ulster because they were forming alliances with the Irish, and ordered Leinster (eastern Ireland) to be carved into counties. For the next 50 years, the Normans attacked more Irish Kings and took over Cork and Ulidia (the kingdom around present-day Belfast). In 1235, the Norman family De Burgos forcibly removed the native Irish Lords from parts of Connaught and founded Galway town. By 1250, the Normans controlled three quarters of Ireland; western Ulster, Kerry, Clare and western Galway being the only predominantly Irish-ruled areas left.

The Normans also brought a new system of government (feudalism) and set up more advanced religious and legal practices. They also built magnificent castles all over Ireland (eg Cashel and Carrickfergus) and founded many towns in the southern half of the island. In this sense at least, the Irish under Norman rule probably enjoyed a higher standard of living than they ever had previously.