Kingdom of Leinster

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 Labhradh Longseach 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).

The Dying Gaul. According to Irish tradition, the name Leinster derives from the spears carried by the Gaul mercenaries brought back by the exile Labhradh Longseach to secure his kingdom.

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. Here Labhradh came across fearsome Gallic warriors known as the Gaesatae, who have been described as wielding spears and shields, rushing into battle naked. They were famous for some impressive military feats in their clashes with the Romans, such as those at the Battles of Faesulae, Telamon and Clastidium with Rome. The Gaesatae were immortalised in artistic form in the Dying Gaul sculpture, an ancient Roman marble held in the Capitoline Museums.

After three decades of hostility, Cobthach and Labhradh made peace, with the latter returning to Ireland. As a reward for recognising his great-uncle as High King of Ireland, Labhradh was rewarded with Gailian, the territorial basis of what would become Kingdom of Leinster. Labhradh did not return to Ireland alone however, he brought with him some military backup in the form of a band of Gaulish mercenaries from the Gaesatae. The kingdom of "Gailian" would soon be renamed as the Kingdom of Leinster (Laighin in Irish), a term which etymologically refers to the broad-pointed spears of the Gaulish mercenaries which Labhradh brought to protect the realm. 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. The familiar pattern of fratricide continued as Labhradh was killed by his own nephew Meilge Molbthach. 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.

The king was also raided and invaded by Viking forces from the 8th century onwards, and by the 9th century, Dublin had become a major Viking settlement and trading center. Some of the Christian monasteries within the kingdom were sacked by them such as St Mullin's.

In the late 10th century, a dynasty known as the Uí Cheinnselaig emerged as the dominant force in Leinster.

The most famous king of this dynasty was Diarmait Mac Murchada, who ruled from 1126 to 1171. Diarmait was a powerful and ambitious king who sought to expand his territory and influence, and he was involved in several wars and alliances with other Irish kings.

In 1166, however, Diarmait was expelled from his kingdom by the High King of Ireland, Ruaidrí Ua Conchobair. Diarmait fled to England, where he sought the support of King Henry II, and in 1170, he returned to Ireland with a force of Norman knights. With their help, he was able to reclaim his kingdom and become even more powerful than before.

Diarmait had several sons, one of whom was named Donnchad mac Murchada. Donnchad became the king of Leinster after his father's death in 1171, but he was forced to share power with his half-brother, Conchobar Mac Murchada. Donnchad's descendants became known as the Kavanagh dynasty, and they continued to hold power in Leinster for centuries.

Art Óg Mac Murchadha Caomhánach, King of Leinster in the late 14th century revived the fortunes of the kingdom and clashed with Richard II Plantagenet, King of England.

The Kavanaghs were known for their fierce resistance to the Lordship of Ireland associated with the Kingdom of England, and they frequently clashed with the forces of the Pale in the late medieval period. One of the most famous Kavanagh kings was Art MacMurrough-Kavanagh, who ruled from 1375 to 1417. Art was a powerful and charismatic leader who was known for his military prowess and his fierce determination to defend his kingdom from English encroachment.

Leinster remained a powerful kingdom throughout the medieval period, but it gradually lost its independence as the English Crown extended its authority over Ireland.[6] In the 16th century, Leinster was incorporated into the English-controlled Province of Leinster as part of the Kingdom of Ireland, which included much of eastern Ireland.

Today, Leinster is one of the four provinces of Ireland, and it is home to several of Ireland's largest cities, including Dublin, Kilkenny, and Wexford.