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.

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.

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.

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.

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] The period in which Labradh was exiled is dated to around the 3rd century BC, when the Roman–Gallic wars were then raging as the Transalpine Gauls were starting to push over the Alps into Etruria, towards the Roman Republic.

[7] After biding his time, Labhradh eventually waged a war on his great-uncle, winning the High Kingship of Ireland and avenge the murder of his progenitors, burning Cobthach and his followers to death at an iron house in Dind Ríg.

Eventually, after several more generations, the family lost control of the High Kingship, with the rise of the Connachta, also a Heremonian tribe, who descend from Óengus Tuirmech Temrach.

Leinster remained a powerful kingdom throughout the medieval period, but it gradually lost its independence as the English Crown extended its authority over Ireland.

By the early 12th century, the Synod of Ráth Breasail took place which moved the organisation of the Church in Ireland from one based on monasteries, to the diocesian model popular throughout the rest of Latin Christendom.

Until the Synod of Kells in 1152, the small walled Norse enclave at Dublin had the unusual distinction of being part of the Province of Canterbury unlike the rest of the Church in Ireland, while Glendalough was associated with the Gaels of Leinster in the same area.

A couple of late examples of positions held by Leinstermen, include St. Lorcán Ó Tuathail, the Archbishop of Dublin, previously the Abbot of Glendalough, who was involved in implementing the Gregorian Reform in Ireland and took part in the Synod of Cashel.

Another native Irish cleric in Leinster, born in the following generation, Ailbe Ó Maíl Mhuaidh, Bishop of Ferns, would attend the Fourth Council of the Lateran in Rome under Pope Innocent III.