His issue unresolved, he gained the military support of the Richard de Clare, 2nd Earl of Pembroke (otherwise known as "Strongbow"), thus initiating the Anglo-Norman invasion of Ireland.

In exchange for his aid, Mac Murchada promised Strongbow the hand in marriage of his daughter Aoife and the right to succeed to the Kingship of Leinster.

Henry II then mounted a larger second invasion in 1171 to ensure his control over Strongbow, resulting in the Norman Lordship of Ireland.

He had two wives (as allowed under the Brehon laws), the first of whom, Sadhbh Ní Faeláin, was mother of a daughter named Órlaith who married Domnall Mór, King of Munster.

His second wife, Mór ingen Muirchertaig, was mother of Aoife / Eva of Leinster and his youngest son Conchobar Mac Murchada.

This was opposed by the then High King of Ireland, Toirdelbach Ua Conchobair (Turlough O'Conor) who feared (rightly) that Mac Murchada would become a rival.

Toirdelbach sent one of his allied kings, Tigernán Ua Ruairc (Tiernan O'Rourke) to conquer Leinster and oust the young Mac Murchada.

As king of Leinster, in the years 1140–70 Diarmait commissioned the Irish Romanesque buildings of: He sponsored convents (nunneries) at Dublin (St Mary's, 1146), and around 1151 two more at Aghade, County Carlow and at Kilculliheen near Waterford city.

The abbey of St. Mary Del Hogge in Dublin was named after the Hoggen Green or Haugr meaning gravesite in old Norse.

He marched his forces into Leinster and, with the mediation of the Church, the commanders of the two armies began negotiations at Ferns, Diarmait's political base.

Some historians maintain that the treaty with Ua Conchobair included a secret agreement whereby Diarmait undertook to bring in no more foreign mercenaries and to send away Robert FitzStephen and his men as soon as Leinster was subdued.

Emboldened by these victories, he sent Robert FitzStephen to the assistance of his son-in-law, Domnall Mór Ua Briain, the King of Thomond.

In the opinion of some historians, Mac Murchada's plans may have been limited to the recovery of his throne; only later when the superiority of the mercenary arms had overawed the Gaelic nobility of Ireland did he consider tilting at the high kingship itself.

[10] According to the contemporary Gerald of Wales, he was advised by Robert FitzStephen and Maurice FitzGerald to write to Strongbow requesting assistance.

In his Book of Leinster, Áed seems to be the first to set out the concept of the rí Érenn co fressabra, the "king of Ireland with opposition", later more widely adopted.

Gerald of Wales, a Cambro-English cleric who visited Ireland in 1185 and whose uncles and cousins were prominent soldiers in the army of Strongbow, repeated their opinions of Mac Murchada: Dermot was a man tall of stature and stout of frame; a soldier whose heart was in the fray, and held valiant among his own nation.

Diarmait's male-line descendants included Art Óg mac Murchadha Caomhánach (d. 1417), who revived the kingship of Leinster, and Cahir mac Art Kavanagh (died 1554) who continued to rule parts of Leinster independently of the English until the Tudor conquest of Ireland in the 16th century.

[7] In the play The Dreaming of the Bones by W. B. Yeats, the ghosts of Dermot and Derbforgaill rescue an Irish rebel during the Easter Week rebellion, and reveal that they are bound until an Irishman can forgive them for bringing the Normans to Ireland.