Dermot MacMorrough, is considered the most noted traitor of Irish history, also commonly known as Dermot naNGhall (Dermot of the foreigners) was the King of the eastern Irish province of Leinster who invited King Henry II of England to invade Ireland to assist him in regaining his throne of Leinster and taking the throne of all Ireland, however, in the end, King Henry became the "Lord" (ruler) of Ireland.

After the death of his older brother, Dermot was unexpectedly crowned King of Leinster, this was opposed by the then High King of Ireland, Turloch O'Connor who feared rightly so that MacMorrough would become a rival. King Turloch sent one of his allied cheiftains, Teirnan O'Rourke (a man who lived for battles) to conquer Leinster and oust the young MacMorrough. O'Rourke went on a brutal campaign slaughtering the livestock of Leinster and thereby trying to starve the province's residents.

MacMorrough was ousted from his throne, but was able to regain it with the help of Leinster clans in 1133, afterwards followed two decades of an uneasy peace between the High King Turloch O'Connor and King Dermot. In 1152 he even assisted the High King raid the land of Teirnan O'Rourke who had by then become a renegade.

After the death of the famous High King Brian Boru (Brian 1st) in 1014, Ireland was at almost constant civil war for two centuries. After the fall of the O'Brien family (Brian Boru's descendants) from the Irish throne, the various families which ruled Ireland's four provinces where constantly fighting with one another for control of all of Ireland. At that time Ireland was like a federal kingdom, with four provinces (Ulster, Leinster, Munster and Connaught) each ruled by Kings who were all supposed to be loyal to the High King of Ireland.

In 1166, Ireland's new High King and Dermot's only ally Muirchertach O'Lochlainn had fallen, and a large coalition led by Teirnan O'Rourke (now Dermot MacMorrough's arch enemy) marched on Leinster. O'Rourke and his allies took Leinster with ease, and MacMorrough and his wife were barely able to escape with their lives. He escaped to England where he formed an alliance with King Henry II who helped him organize a mercenary army of Norman and Welsh soldiers to invade Ireland.

In his absence Rauri O'Connor (son of Dermot's former enemy, High King Turloch) had become the new High King of Ireland. MacMorrough planned not only to retake Leinster, but to oust the O'Connor clan and become the High King of Ireland himself. He quickly retook Dublin, Ossary and the former Viking settlement of Waterford, and within a short time had all of Leinster in his control again.

He then marched on Tara (then Ireland's capital city) to oust Rauri O'Connor, Dermot MacMorrough gambled that King Rauri would not hurt the Leinster hostages which he had (including MacMorrough's eldest son, Connor MacMorrough), however O'Rourke forced his hand and they were all killed.

Dermot's army lost the battle and the Norman and Welsh mercenaries whom he had hired soon aided an invasion by England's King Henry II in 1169. MacMorrough lost his will to fight after his son's death, retreated to Ferns and died a few months later. In Irish history today Dermot MacMorrough is written as being a traitor, however technically his intention was never to aid an English invasion of Ireland, but to become the High King of Ireland himself with the help of the English King, he had no way of knowing Henry II's ambitions on Ireland.

Afterwards the Normans conquered Ireland by playing one Irish family off against another, Rauri O'Connor was soon ousted as High King and eventually as King of Connaught and therefore to regain his provincial kingdom like MacMorrough turned to the English. By 1171, England controlled a small territory in Ireland surrounding the city of Dublin known as "the Pale", while the rest of Ireland became divided between Norman and Welsh barons sent by the English, and the various Irish Clans (like the O'Connors who held onto Connaught and the O'Niells who held onto Ulster).

However eventually most of the ruling Norman families began intermarriages with the Irish, allied with Irish clans against England, adopted the Irish language and as the English put it "became more Irish than the Irish themselves" prompting a second English invasion centuries later. Whether Dermot MacMorrough was an actual "Benedict Arnold" or "Vidkun Quisling" (names of infamous American and Norwegian turncoats) can be debated, however when looking at history it appears more likely that he was not a traitor, but another ambitious petty king, whose eyes where bigger than his stomach. Regardless one thing is for sure, after more than 800 years of war between Ireland and England from 1171-1921, starting with his Norman invasion in 1166, Irish history will most likely never forgive Dermot MacMorrough.