The Gaelic Kingdom of Leinster before 1171 is considerably smaller than today's province.

The first part of the name Leinster derives from Laigin, the name of a major tribe that once inhabited the area.

The latter part of the name derives either from the Irish tír or the Old Norse staðr, both of which translate as "land" or "territory".The tribes of Leinster were united by Úgaine Mór (Hugony, the Great), who supposedly built the hill-fort of Dún Ailinne, near Kilcullen, County Kildare.

He is a likely, but uncertain candidate as the first historical king of Laigin (Leinster) in the 7th century BCE.

The kingdom of Laigin is re-founded circa 175/185 following a period of civil wars in Ireland by the legendary Cathair Mor.

Finn Mac Cool, or Fionn mac Cumhaill, was reputed to have built a stronghold at the Hill of Allen, on the edge of the Bog of Allen, in what was then Leinster.In the 4th and 5th centuries, after Magnus Maximus leaves Britain with his legions, leaving a power vacuum, colonists from Laigin settle in North Wales, specifically in Anglesey, Carnarvonshire and Denbighshire.

In Wales some of the Leinster-Irish colonists leave their name on the Llŷn Peninsula, which derives its name from Laigin.

In the 5th century the emerging Uí Néill dynasties from Connacht conque areas of Westmeath, Meath and Offaly from the Uí Enechglaiss and Uí Failge of the Laigin.

Uí Néill Ard Righ attempt to exact the Boroimhe Laighean, or cattle-tribute from the Laigin from that time, in the process becoming their traditional enemies.By the 8th century, the rulers of Laigin have split into two dynasties: Northern Leinster dynasty: Murchad mac Brain (d. 727), King of Uí Dúnlainge, and joint leader of the Laigin Southern Leinster dynasty: Áed mac Colggen (d. 738), King of Uí Cheinnselaig, and joint leader of the LaiginAfter the death of the last Kildare-based King of Laigin, Murchad Mac Dunlainge in 1042, the kingship of Leinster reverts to the Uí Cheinnselaig sept based in the south east, now County Wexford.

This southern dynasty gives all the later Kings of Leinster.