From the book "The Oxford History of Ireland" edited by R. F. Foster, 1992, ISBN 0-19-285271-X, page 28-9:
Originally, Leinster stretched northwards to the Boyne but the land between it and the Liffey was lost to the advancing Uí Néill. The heartland of historic Leinster was the vale of the Liffey, and the valleys of the Barrow and the Slaney. The great dynasties, the winners, were in the plains and river valleys; the losers were on the eastern coastal strip, the Wicklow highlands, and the north-western boglands. In the 8th century Leinster was ruled by the Uí Dúnlainge, a group of dynasties settled in the vale of Liffey and in the plains of Kildare. They were new men: the annals and genealogies show that other groups-whom they had pushed out into the badlands-had held the kingship before them. They had other rivals to the south of them: Uí Chennselaig, who had threatened the vale of the Liffey in the 7th century-but they were foiled and pushed south, where they carved out an important kingdom for themselves in association with Ferns and a number of other monasteries.