Originally an ethnic term, the word "Laigin" refers to the people who dominated the southeast of Ireland and gave their name to the province of Leinster (Coiced Laigen). Medieval sources closely associate the Laigin or "Leinstermen" with the Gaileoin and Domnainn, to whom they were probably related, but regard them as ethnically distinct from the Ulaid, Connachta, and other provincial powers. Although reliable information about their origins and rise to power is lacking, medieval legend suggests that the Laigin came to Ireland from either Britain or Gaul under the leadership of their ancestor Labraid Loingsech and seized control of the province of Leinster some time in the third or fourth century b.c.
By the dawn of the historical period, the Laigin had split into many separate dynasties and spread throughout the province. The most powerful of these—dynasties like the Ui Garrchon, Ui Enechglais, and Ui Failgi— lived in the north and vied for control of the Liffey valley, an area that encompassed Naas, Kildare, and Dun Ailinne, the symbolic center of their provincial kingship. During the fifth and sixth centuries, these dynasties were engaged in territorial wars with the expanding Ui Neill, to whom they ultimately lost possession of Tara and its environs. Conflicts with this dynasty would become a recurrent feature of Laginian history, particularly in the eighth and ninth centuries as the Ui Neill attempted to assert their suzerainty over successive kings of Leinster. By the mid-eighth century, these conflicts in conjunction with numerous internal feuds significantly weakened many of the old northern dynasties such that a new Laginian power structure arose, one dominated by the Ui Dunlainge in the north and the Ui Chennselaig in the south.
From 738 to 1042, the Ui Dunlainge ruled the Liffey valley and maintained an exclusive hold on the provincial kingship. However, by the early decades of the eleventh century, internal disputes, conflicts with the Vikings, and invasions by successive kings of Osraige had crippled the Ui Dunlainge septs, allowing Ui Chennselaig to seize power. Before that time, the latter had enjoyed a measure of independence from Ui Dunlainge in their new homeland around Ferns, though they were wracked by dynastic strife. But with the Ui Dunlainge weakened, they seized the Laigin kingship in 1042 and dominated the province from that point until the Anglo-Norman Invasion. The person responsible for their rise to power was Diarmait mac Maele-na-mbo, who was one of the most powerful kings in Ireland at his death in 1072. In the early twelfth century, the ruling family of Ui Chennselaig adopted the surname Mac Murchada (Mac Murrough), and one of the first kings to bear it was Diarmait Mac Murchada (d. 1171), whose arrangement with Henry II made possible the Anglo-Norman Invasion. Once the English got control of Leinster, the Mac Murchada family would enjoy only occasional bouts of power, as they did under Art Mac Murchada in the late fourteenth century, but it was not until the late sixteenth century that they were completely brought to heel.
