In ancient times Leinster was occupied by five major Fir-Bolg tribes, of which the tribe of Laigin rose to dominance.. The Fir Bolg may be related to the Belgae, but this has not been firmly established. The tribes of Leinster were united by Úgaine Mor [Hugony "The Great"], who built the hill-fort of Ailinne [Knochawlin], near Kilcullen, County Kildare, and is a likely, but uncertain candidate as the first historical King of Laigin [Leinster] in the 7th century BC. The "ster" portion of the toponym comes from a Nordic word for state, as the Vikings dominated and held, among other towns, Dublin and Waterford for a period of time.

The kingdom of Laigin [Leinster] was re-founded circa 175/185 AD following a period of civil wars in Ireland by the legendary Cathair Mor. Finn Mac Cool (Fionn Mac Cumhaill) was reputed to have built a stronghold at the Bog of Almhainn, in what was then Leinster.

In the fourth and fifth centuries, after Magnus Maximus left Britain with his legions, leaving a power vacuum, colonists from Laigin settled in north Wales; Specifically Anglesey, Carnarvonshire, and Denbighshire. In Wales some of the Leinster-Irish colonists left their name on the Llyn peninsula, which derives its name from Laigin. By the eighth century, Leinster, had split into two dynasties:

After the death of the last Kildare-based King of Laigin, Murchad Mac Dunlainge in 1042, the Kingship of Leinster reverted to the Uí Cheinnselaig sept based in the south east (southern dynasty) which comprise later Kings of Leinster. Some contend it was named after a powerful Welsh colony, meaning Land of the men of the Llyn Peninsula

Today, made of twelve counties, it encompasses the old province of Mid (mostly now in modern-day County Meath and County Westmeath). Also in it are County Longford and the Annally and Lusmagh parishes of County Offaly, formerly of Connacht, and County Louth, formerly of Ulster. The borders were redrawn by Cromwell for administration and military reasons. The last major boundary changes occurred with the formation of County Wicklow (1603-1606), from lands in the north of Carlow (which previously extended to the sea) and most of southern Dublin. Later minor changes dealt with "islands" of one county in another.