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.

During the 5th to the early 8th centuries, the Kingship of Leinster was contested by various different branches of the Laighín, including the Uí Cheinnselaig (ancestors of the Mac Murchada and Ó Tuathail), the Uí Bairrche (ancestors of the Mac Gormáin), the Uí Máil (ancestors of the Ó Conchobhair Uí Failghe) and others. Following this period, until the 11th century, Leinster was mostly contested between two branches of the Uí Dúnlainge kinship, represented by what are today the families of Ó Tuathail and Ó Brion. In the 9th century, the Laighín also regained control of Osraige but it remained a largely independent realm under the Mac Giolla Phádrag.

Leinster had to contend with raids from the Vikings under the Uí Ímair from the 9th century onwards, who established themselves at Dublin and Wexford. As part of these Gaelic-Viking battles, Murchad mac Diarmata, King of Leinster took control of the Kingdom of Dublin and the Kingdom of Mann and the Isles (what is now the Isle of Man and the Scottish Hebrides) for a brief period. His father, Diarmait mac Máel na mBó, was the first Leinster High King of Ireland in centuries. This reversal in fortunes was brief, however, with the 12th century Norman invasion of Ireland seeing Leinster closely caught up in the affairs with Diarmait mac Murchada. In the longterm Leinster lost territories to the Normans, which became the Pale as the administrative centre of the Kingdom of England's Lordship of Ireland, as well as the Earldom of Kildare to the FitzGerald dynasty. This did not mean the end of Leinster, however and the kingdom continued to control much of what is today County Wexford and County Carlow and parts of County Wicklow until the early 17th century when it became part of the Tudor Kingdom of Ireland, later being revived as the Province of Leinster.

The name of the Kingdom of Leinster is derived directly from the tribal name Laigin, a group of Irish Gaels who all claim descent in the male paternal line from Érimón. Other branches of the Heremonians include the Érainn, Connachta, Uí Néill, Clan Colla, Uí Maine and the Dalcassians (though they invented genealogies to deny this). Labhraidh Loingseach is sometimes cited as the progenitor of the Laigin but his direct paternal ancestors had been High Kings of Ireland and Kings of Tara for some time before he emerged on the scene. A late medieval text in Middle Irish named Cóir Anmann (known in English as the "Fitness of Names" or the "Elucidation of Names") gives an etymology for the term Leinster. It claims that the name derives from the word laigin ("spears"), in reference to the large spears carried by "2,200" Gaulish mercenaries which Labhraidh Loingseach brought back from the Continent with him and hosted at Cobthach Cóel Breg's house at Dind Ríg. His descendants were subsequently known as the Laigin ("Leinstermen").

The kingdom was founded by the Laighin, a Heremonian tribe of Irish Gaels: that is to say the rulership group claimed descent in the paternal line from Érimón, son of Míl Espáine (though other groups of Irish Gaels also lived within the territory of the Kingdom of Leinster) and has provided many early High Kings of Ireland. The branch which became the Laighin had been in power struggle with the Érainn kinship group of the Corcu Loígde, also known as the Dáirine, a fellow Heremonian tribe, for hegemony in Ireland.

According to traditional Irish history found in the Annals of the Four Masters, the earliest King of Leinster of note was Úgaine Mór, who was a High King of Ireland and had as his wife and queen, Cessair Chrothach, supposedly Gaulish woman (that is to say a member of a Celtic group from Continental Europe). The histories of this period describe a lot of internal bloodshedding between different members of the royal family as they ruthlessly vied for the High Kingship of Ireland: Úgaine Mór was supposedly killed by his own brother Bodbchadh, who subsequently reigned as High King of Ireland for a mere day and a half until himself being killed by his nephew Lóegaire Lorc, son of Úgaine Mór. He was not the only child of Úgaine Mór, as he supposedly had at least 25 offspring (twenty-two sons and three daughters) and is said to have parcelled out vast swathes of land in Ireland among his children.

Lóegaire's older brother Cobthach Cóel Breg was also an ambitious man and after seeking advice from a Druid, slew his brother in a treacherous manner. Cobthach carried this out by pretending to be on his death bed and when his brother Lóegaire Lorc lent over his body in grief, jumped up and stabbed his own brother to death. After seizing the High Kingship for himself, Cobthach proceeded to have Lóegaire Lorc's son Ailill Áine poisoned to death and forced his grandson Labhraidh Loingseach into exile in Gaul, on the European Continent. Stories of the period relate that, before leaving for the Continent, Cobthach supposedly forced his great-nephew Labhradh to eat part of his slain father and grandfather's hearts (in an act of involuntary cannibalism), as well as a mouse. Labhradh stayed out his exile on the European Continent for thirty years, living principally amongst the Gauls (his grandmother, queen Cessair Chrothach, was supposedly Gaulish).^{[citation needed]}