The Laigin, modern spelling Laighin (Irish pronunciation: [ˈl̪ˠaːjɪnʲ] Lain), were a population group of early Ireland who gave their name to the province of Leinster (IrishCúige Laighean, province, literally fifth, of the Laigin; the English word "Leinster" derives from Irish Laigin and Old Norsestaðr, place, territory). Laigin is a plural noun, indicating an ethnonym rather than a geographic term.
The use of the word cuige, earlier cóiced, literally "fifth", to mean "province", implies the existence at some point in prehistory of a pentarchy, whose five members are believed to have been population groups the Laigin, the Ulaid (Ulster) and the Connachta (Connacht), the region Mumu (Munster), and the central kingdom of Mide. Archaic poems found in medieval genealogical texts distinguish three groups making up the Laigin: the Laigin proper, the Gáilióin, and the Domnainn, who are likely related to the British Dumnonii.
Early Irish historical traditions credited the founding of the Laigin to the legendary High KingLabraid Loingsech. His grandfather, Lóegaire Lorc, had been overthrown by his own brother, Cobthach Cóel Breg, and Labraid forced into exile. After a period of military service on the continent, Labraid returned to Ireland at the head of an army, known as Laigin after the broad blue-grey iron spearheads (láigne) they carried. The Lebor Gabála Érenn dates Labraid's accession to 300 BC. Modern historians suggest, on the basis of these traditions and related placenames, that the Laigin were a group of invaders from Gaul or Britain, who arrived no later than the 6th century BC, and were later incorporated into the medieval genealogical scheme which made all the ruling groups of early Ireland descend from Míl Espáine. Placenames also suggest they once had a presence in north Munster and in Connacht.
See O'Rahilly's historical model for a summary of the Laigin at the height of their power in Ireland. They may have conquered approximately half the island.