The Gaelic Kingdom of Leinster before 1171, considerably smaller than the present-day province, usually did not include certain territories such as Meath, Osraige or the Viking cities of Wexford and Dublin.

Úgaine Mór (Hugony the Great), who supposedly built the hill fort of Dún Ailinne, near Kilcullen in County Kildare, united the tribes of Leinster.

Circa 175/185 AD, following a period of civil wars in Ireland, the legendary Cathair Mor re-founded the kingdom of Laigin.

[8] Uí Néill Ard Rígh attempted to exact the Boroimhe Laighean (cattle-tribute) from the Laigin from that time, in the process becoming their traditional enemies.

The other three provinces had their own regional "Presidency" systems, based on a Welsh model of administration, in theory if not in fact, from the 1570s and 1580s up to the 1670s, and were considered separate entities.

The provincial borders were redrawn by Cromwell for administration and military reasons, and the Offaly parishes of Annally and Lusmagh, formerly part of Connacht, were transferred in 1660.

The last major boundary changes within Leinster occurred with the formation of County Wicklow (1603–1606),[13] from lands in the north of Carlow (which previously extended to the sea) and most of southern Dublin.

The province is divided into twelve traditional counties: Carlow, Dublin, Kildare, Kilkenny, Laois, Longford, Louth, Meath, Offaly, Westmeath, Wexford and Wicklow.