Ireland in the 12th century was a politically volatile, unstable place made up of several kingdoms. Kings, chieftains, and lords were frequently at war, with lands and areas changing hands, bringing the people living off those lands under different overlords.
One reason for the near-constant warfare was the way a ruler's title and position was passed on when he died. Rather than passing automatically to the first-born male son or closest male relative, the title and position could go to whichever relative was deemed the strongest or most capable. This usually meant that entire extended families would go to war amongst themselves. Splitting of domains was not uncommon, and alliances would be forged and broken with other families in the process.
Trade continued to flourish particularly from port towns such as Dublin, Limerick, Waterford and Cork. These port towns, originally controlled by Norse kings & populated by people of Scandinavian and Gaelic descent, were every bit as vulnerable to attack and invasion as the rest of the countryside.
During the 1160s, former King of Leinster Diarmait Mac Murchada lost his kingdom. He needed military support to get it back, and travelled to Wales looking for allies. Wales by this time was largely under the control of Anglo-Normans. He got the support of Strongbow, 2nd Earl of Pembroke, who agreed to come to Ireland to fight for him with a large army. In exchange, Mac Murchada married off his daughter Aoife to Strongbow, along with the promise that he would inherit the kingdom of Leinster after Mac Murchada's death.
In 1169, Strongbow landed in Co Wexford with a large army. Over the next few months, they besieged towns and waged war over much of Leinster until Mac Murchada reclaimed the throne.
In 1171, Mac Murchada died suddenly. Strongbow declared himself king of Leinster. Many nobles who had fought with him were granted lands in Leinster.
Henry II of England became concerned that Strongbow - legally one of his subjects - might set up an independent kingdom in Ireland. In October 1171, Henry II landed in Waterford with a huge army - this was the first time that a reigning King of England set foot on Irish soil.
The army did not fight much; the Norman warlords submitted to Henry II and handed over their conquered territory to him. In addition to this, 15 Gaelic kings and chiefs willingly submitted to him - probably in the hopes that this would stop the Norman warlords trying to invade their lands. The hierarchy of the church also submitted to Henry.
Henry also organised the synod of Cashel, where Irish church practices were brought into line with those of England - at least in Leinster.
Throughout this period, warfare continued as before among different factions - the one difference being that now there were Normans in the mix, fighting against various Gaelic and Norse chieftains.
