Leinster, or Coiced Laigen (the Fifth of the Laigin), is one of the ancient provinces of Ireland. The dominant inhabitants were the Laigin, who believed they came from Gaul in prehistoric times. There were links across the Irish Sea in the early centuries c.e., for the Lleyn peninsula in Wales takes its name from the Laigin. Traditions preserved in historical tales and the annals show that the Laigin controlled a vast territory before the sixth century, including much of Brega and Mide; this land was ultimately lost to the Ui Neill, but there were Laigin kings of Tara before the reign of Niall Noigiallach. By the seventh century, Leinster’s boundaries extended from the valley of the Liffey westwards to the Slieve Bloom Mountains, then southward around the highlands of Osraige and down the Barrow valley to the sea. It was divided into north Leinster, Laigin Tuathgabair, and south Leinster, Laigin Desgabair, and kings in each area enjoyed a considerable degree of independence. The main settlement was in the valleys of the Liffey, Barrow, and Slaney, and in the plains of Wexford and Kildare. In the latter area is the hillfort of Dun Ailinne (Knockaulin), a site comparable to Tara or Emain Macha, and an important centre of the early Leinster kingship.
From the seventh century to the twelfth, the leading dynasties were Ui Dunlainge in the north and Ui Chennselaig in the south, but other Laigin groups had been paramount in Leinster beforehand. Ui Garrchon were settled in the Liffey valley, and at least two of their kings of Leinster fought against the Ui Neill around the end of the fifth century. Ui Enechglaiss also provided early provincial kings. These dynasties suffered as a result of Ui Neill expansion and were ousted from their fertile lands by Ui Dunlainge, resettling east of the Wicklow Mountains. Ui Dunlainge then strove for power with the Ui Mail. A few Ui Mail dynasts succeeded to the provincial kingship, the last being Cellach Cualann (d. 715). Subsequently, Uf Mail were deprived of both the Leinster kingship and the fertile lowlands by Ui Dunlainge. Also significant were Ui Failge, who occupied the boggy lands at the headwaters of the Barrow. They had previously ruled a larger territory, but also suffered from Ui Neill encroachments. The Loigis were settled southeast of Slieve Bloom and were totally unrelated to the Laigin. They had the status of favored vassals of the Leinster kings, principally for their role as defenders of this border area. A further non-Laigin people were the Fothairt, who were scattered throughout Leinster, and to whom St. Brigit belonged.
In south Leinster Ui Chennselaig were dominant by the eighth century, but here too were earlier Laigin dynasties. Ui Bairrche occupied lands in Carlow, but were divided by Ui Chennselaig expansion so that one branch remained in the middle reaches of the Barrow valley, and another moved southwards to the Wexford coast. Also in south Leinster were groups of Fothairt, associated with Ui Bairrche, and a people called the Benntraige.
After 738, Ui Dunlainge excluded all other peoples from the Leinster kingship. Their ascendancy was gradually eroded by the Ui Neill, who regularly tried to gain the submission of Leinster’s kings. The depredations of Viking incursions also had a destabilizing effect on the Ui Dunlainge hegemony; its decline was accelerated by the interference of the kings of Munster and Osraige. The establishment of the Viking settlement at Dublin in 841 was of undoubted significance, creating new maritime links, and providing a center of wealth on Leinster’s doorstep. By the eleventh century, Dublin and its hinterland, Fine Gall, were closely linked to Leinster, and Irish kings strove for dominance over the rulers of the city and control of its resources. Meanwhile, Leinster had become a significant factor in the struggle for high kingship between Ui Neill and the Dal Cais. Domination by their kings and the kings of Osraige fatally undermined Ui Dunlainge authority, and in 1042 Diarmait mac Maele-na-mBo of Ui Chennselaig took the Leinster kingship. Though ultimately unsuccessful in his challenge for the high kingship, he achieved more than any previous king of Leinster. Control of Dublin had been a key factor in his success, and this lesson was not lost on his contemporaries. His descendants, the Meic Murchada (Mac Murroughs) retained the provincial kingship and played an important role in later interprovincial struggles. A pivotal role was played by Diarmait Mac Murchada, whose expulsion from and return to Ireland led to the Anglo-Norman Invasion. Subsequently, a considerable number of colonists entered Leinster, notably the de Clares and Fitzger-alds, and established lordships in Kildare, Carlow, and Wexford.
The Mac Murroughs retained a degree of power in south Leinster, though hemmed in by the English. This changed with the career of Art Mor Mac Murchada Caomhanach from the 1370s to 1416, who created a solid kingdom in Carlow and northern Wexford, and Mac Murroughs succeeded to the title "king of Leinster" down to the sixteenth century. Ui Dunlainge, represented principally by the families of Ua Broin (O’Byrne) and Ua Tuathail (O’Toole) were driven by the invaders from their lands into the Wicklow Mountains, thus suffering the fate they had inflicted on Ui Garrchon and Ui Mail centuries earlier. However, both families were able to create enduring Gaelic lordships in the highland fastnesses. The O’Tooles were often enemies of the English crown, but ultimately in 1541 Tairrdelbach Ua Tuathail submitted to Henry VIII of England. The O’Byrnes had lands in the southern Wicklow Mountains and in Crioch Branach (O’Byrne Territory) on the coast. They too fought against the English and other Gaelic lords, but by the later sixteenth century the coastal areas were largely in the orbit of the colonial administration. One branch of the family, the lords of Crioch Raghnall (Raghnall’s Territory) in the Wicklow Mountains, resisted the English to the end of the sixteenth century, notably in the person of Fiach Ua Brain, whose death in 1597 heralded the end of Gaelic Leinster.
