From Irish Roots, 1996, #4, p21, 'Surnames of County Laois':
The O'Moores dominated the political life of Laois until after the arrival of the Normans. . . The principal residence of the O'Moores was Dunamase, a remarkable castled crag, where the ruined fortress may still be seen. . . On the arrival of the Normans in the 12th century, Dunamase was held by Dermot McMurrough, the King of Leinster. His alliance through the marriage of his daughter Eva to Richard de Clare, surnamed Strongbow, brought the whole province of Leinster under Anglo-Norman domination and presented the beginning of the end for the O'Moores and their subsidiary septs. While their demise was protracted, the ultimate defeat of the old Gaelic clans was never in doubt. There is a picture of Dunamase in the magazine.

From Irish Roots, 1994 #4, page 15, 'Surnames of County Leitrim':
One of the most momentous events in Irish history, the elopement of Dervorgilla, took place on O'Rourke territory. sometime around 1160 Tiernan O'Rourke's wife, Dervorgilla, eloped with Diarmuid MacMurrough. In 1166 O'Rourke marched on Leinster, where Diarmuid was king. MacMurrough sought aid from King Henry II of England and he, with the blessings of an English Pope, Adrian IV, sent Strongbow. This led effectively to the English occupation of Ireland.

From the book "The Oxford History of Ireland" edited by R. F. Foster, 1992, ISBN 0-19-285271-X, page 43:
Muirchertach Mac Lochlainn was king of the Uí Néill. He allied himself with Dermot MacMurrough (Diarmait Mac Murchadha), king of Leinster. His main opponent was Rory O'Connor, king of Connacht. In common with all the greater kings of the 12th century, he realized that Dublin was the capital of Ireland. He and MacMurrough (MacMurchadha) did their best to hold it. When Mac Lochlainn fell, the Dubliners did their bit in unhorsing Mac Murrough, and they and O'Connor's allies drove him out. He appealed for help to King Henry II of England and changed the course of Irish history in doing so. The invitation became an invasion, which opened up Ireland politically to expansive Anglo-French feudalism.
page 46:
Dublin, with its central position, its agriculturally rich hinterland and commercially successful port, was suited to act as capital not only for Ireland as a whole, but for the provincial kingdom of Leinster, then ruled by Dermot MacMurrough. He was called King of Dublin just as the O'Brien and MacCarthy kings of north and south Munster were then known as the kings of Limerick and of Cork respectively.
page 47:
The Dubliners themselves, who had killed Dermot's father and buried his corpse along with that of a dead dog under the floor of their assembly-hall, apparently preferred the prestige and comparative independence of association with a distant national ruler [in England] to the domination of a local tyrant [the MacMurrough kings of Leinster]. A conflict of interests had been avoided in earlier years when the northern high-king, Muirchertach MacLochlainn (d. 1166), repeatedly allied with Dermot MacMurrough to subject Dublin to their joint authority. However, no such compromise was available to the new high-king, Rory O'Connor. Based as he was in the province of Connacht, Rory's most strategically placed ally was the one-eyed king of Bréifne, Tighearnán O'Rourke (O Ruairc), an implacable enemy of MacMurrough. In 1152 the rivalry between these two kings for overlordship of the agriculturally rich but politically weak province of Meath had found expression in Dermot's abduction of O'Rourke's wife, the Meath princess Dervorgilla (Dearbhfhorgaill). When O'Connor came to power in 1166, O'Rourke ensured that MacMurrough was not merely forced to submit and give hostages to the high-king, but was subsequently dethroned. Since the Dubliners were fighting for O'Rourke and O'Connor on this occasion, Dermot was forced to travel overseas in search of mercenary troops to retrieve his position.
The idea of invading Ireland had been discussed from time to time at the English court during the reigns of William the conqueror and Henry I, and immediately after the accession of King Henry II. The secretary of the archbishop of Canterbury was sent as an envoy of Henry II to the English pope, Adrian IV.
page 48:
As a result, the pope invested Henry and his successors with the right to rule Ireland, since, by virtue of a clause in the Donation of Constantine, the pope was held to be lord of all the islands of the sea. Henry II did not divert his forces towards Ireland because at the outset of his reign his hold on England itself was insecure. When King Dermot approached him for help in 1166, Henry II was not prepared to become actively involved. He accepted homage and fealty from the Irish ruler and authorized his own subjects to come to MacMurrough's aid.
Using Bristol as his base, Dermot initially recruited Richard fitzGilbert de Clare, earl of Pembroke, and a group of Cambro-Norman barons including the half-brothers Maurice FitzGerald and Robert FitzStephen, both sons of the Welsh princess Nesta. These latter were promised Wexford and two adjoining cantreds for their services, while de Clare, better known as 'Strongbow', was offered Dermot's daughter, Aoife in marriage and promised the whole province of Leinster on Dermot's death. Between 1169-1171 the Cambro-Normans not only reconquered all Leinster including Dublin for Dermot, but invaded the neighboring province of Meath and harried Tighearnán O'Rourke's kingdom of Bréifne.

Dermot MacMurrough (or Dermod naNGhall, meaning Dermot, king of the Foreigners) was born in 1110 AD. At an early age he was fostered out to a minor family on the border of Leinster, in the neighboring state of Ossory and here he grew to manhood. At age 16, upon the unexpected death of his older brother (the king of Leinster), he was elected king of Ui Cinnsealaigh. What followed was the turning point in Irish history.
Upon receiving the kingship, Dermot also became king of Leinster, like his brother before him. This the High King of Ireland, Turlough O'Connor of Connaught, opposed, so he sent a neighboring chieftain to subjugate Leinster--Tiernan O'Rourke, a man who loved battle. Among the three sacred laws
of Ireland, the one called Daire's Law specifically forbade the killing of cattle by an enemy for by killing cattle, you were forcing the people of that land to starve because dairy products were their sole food source. O'Rourke killed the cows of Leinster.
It took years for Dermot to regain the throne of Leinster, but finally, by 1133 he had succeeded and now began to expand his power. He raided in Ossory, then sacked Waterford, like his great-grandfather, Dermot Mac Mal namBo, before him. The following two decades were mostly peaceful for Dermot, he avoided many of the wars the other four provinces were waging on one another, but in 1152, he helped Turlough O'Connor raid Tiernan O'Rourke's land.

After O'Rourke's land was destroyed and his castle burned, the armies left for their respective provinces. As Dermot was traveling through Meath to return to Leinster, the King of Meath told him that Dervorgilla, O'Rourke's wife and also the King of Meath's sister, would run away with him. Dermot turned around and picked up Dervorgilla with all her furniture and cows, then returned home. When O'Rourke discovered his wife had been taken, he was furious.

This 'abduction' made Dermot Tiernan O'Rourke's most bitter enemy. After only a year, Dermot was forced to give Dervorgilla back, but O'Rourke never forgave. In 1166, when Ireland was ablaze in war and Dermot's ally, the High King Muirchertach O'Lochlainn, had fallen, O'Rourke joined together a number of other chieftains and raided Leinster...O'Rourke for revenge, the rest for plunder. Dermot barely escaped with his life and sailed for England. Dermot was not through.

MacMurrough gathered a force of Norman and Welsh fighting men and returned to Ireland. In quick succession he defeated Ossory, Waterford, and then Dublin, so reclaiming the kingship of Leinster, but he was not satisfied. He marched on the High King of Ireland, Rory O'Connor (Turlough's son), and demanded the High King's submission. Dermot gambled that Rory would not hurt the Leinster hostages he had, Dermot's son and nephew. Rory hesitated, then O'Rourke forced his hand.

The bodies of Dermot's son, Conor, and nephew were delivered to him in a sack like a bullock would be delivered to market. Dermot lost the will to fight. His army disbanded and he returned to Ferns where, a few months later, he died.