NBGreenDay said:
ah mac Murchada had returned from england with a force of welsh and normans to reclaim the High Kingship of Ireland. In response, Conchobair killed the hostages he had, mac Murchada's son and nephew, and delivered their bodys to mac Murchada. Mac Murchada lost the will to fight and disbanded to the ferns.
Well, not quite. Diarmait mac Murchada was king of Laigin (Leinster). According to Giraldus Cambrensis, he abducted the wife of Tigernán Ua Ruairc, the lord of Bréifne and Conmaicne. Tigernán formed an alliance Ruaidrí Ua Conchobair, King of Connachta and High King of Ireland; and Diarmait was expelled from his kingdom.
Now, if it had all ended there, history would be very different. Instead, Diarmait went France to see Henri II of England, offered his homage, and asked for help being restored to his "kingdom". Henri accepted to help him and gave him letters patent, which contained the following:
"When you receive this present letter, be advised that we have admitted to our most intimate grace and favor Dermit, Prince of Lagen. Wherefore, if any person from within our wide dominions wishes to help in restoring him, as having done us fealty and homage, let him know that he has our goodwill and permission to do so"
Diarmait obtained the help of Robert FitzStephen and Maurice FitzGerald, by promising Maurice and Robert the city of Wexford and surrounding area. Diarmait crossed over to Ireland alone in 1169, FitzStephen and FitzGerald a little later in the year. They, together, subdued Wexford and Osraige...and later Dublin, along with other palces; they also saw off the army of Ruaidrí Ua Conchobair (although without engaging it).
Diarmait, if we believe Giraldus, then raised his ambitions, and began to aim at the kingship of Connachta and the High-Kingship of Ireland. The problem was manpower. Diarmait sent a letter to Richard FitzGilbert de Clare (aka "Strongbow"), nominal Earl of Pembroke, offering Richard his daughter in marriage (and succession to his kingdom) in return for help. Strongbow agreed. Their joint forces beat off invasions by the Norwegians and the "Men of the Isles". Not long after, Diarmait died.
Soon (still 1171) Henri II himself began to get worried about the success of his vassals, and ordered them all to withdraw. The Cambro-Norman adventurers in Ireland sent representatives to his court to appeal. Henri decided to visit the Island, and when he arrived, he received the submission of almost every ruler in the island outside of Ulaid. He also received the submission of Ruaidrí Ua Conchobair, which, as he was High King of Ireland, meant in feudal terms the submission of all Ireland. Henri began issuing laws and depriving his old vassals of independent power. Henri also obtaisn the support of the Papal Legate and the Papacy (in return for one penny per household in Ireland).
However, Henri was forced to leave because his sons were rebelling. In 1177, Henri sent his son Jean to rule the island as a "Lordship". Strongbow had died in 1176, leaving his rights as king of Laigin to Henri.
Diarmait's conflict with Ruaidrí Ua Conchobair, hence, led to the Ango-Norman (or/and rather, Cambro-Norman) invasion of Ireland. It led to the first English foothold in Ireland, and since, the English have ruled either all of Ireland or bits of it, since...to the present day. It might be argued that Diarmait was a kind of Irish Vortigern.