The Kingdom of Leinster (Irish: Ríocht Laighean) was a kingdom of Gaelic Ireland which existed in the east of the island from the Irish Iron Age until the 17th century Early Modern Ireland. According to traditional Irish history found in the Annals of the Four Masters, the kingdom was founded as the territory of the Laighin, a Heremonian tribe of Irish Gaels. Some of the early kings of Leinster were also High Kings of Ireland and Kings of Tara, such as Úgaine Mór, Labraid Loingsech and Cathair Mór.
The Leinstermen had originally achieved hegemony in Ireland to the detriment of the Ulster-based Érainn, another group of Irish Gaels, but eventually lost out to their kinsmen the Connachta. This fall from power had lasting consequences in terms of territory for Leinster, as the Southern Uí Néill carved out the Kingdom of Meath to the north, and control of Osraige to the west was lost to the Corcu Loígde, becoming part of the Kingdom of Munster. The kingdom had different borders and internal divisions at different times during its history.