Leinster (// — Irish: Laighin / Cúige Laighean — pronounced / ) is one of the Provinces of Ireland situated in the east of Ireland. It comprises the ancient Kingdoms of Mide, Osraige and Leinster. Following the 12th-century Norman invasion of Ireland, the historic fifths of Leinster and Mide gradually merged, mainly due to the impact of the Pale, which straddled both, thereby forming the present-day province of Leinster. The ancient kingdoms were shired into a number of counties for administrative and judicial purposes. In later centuries, local government legislation has seen further sub-division of the historic counties.
Leinster has no official function for local-government purposes. However, the province is an officially recognised subdivision of Ireland. It is listed on ISO 3166-2 as one of the four provinces of Ireland and "IE-L" is attributed to Leinster as its country sub-division code.
Leinster had a population of 2,630,720 according to the preliminary results of the 2016 census, making it the most populous province in the country. The traditional flag of Leinster features a golden harp on a green background.
The Gaelic Kingdom of Leinster before 1171, considerably smaller than the present-day province, usually did not include certain territories such as Meath, Osraige or the Viking cities of Wexford and Dublin. The first part of the name Leinster derives from Laigin, the name of a major tribe that once inhabited the area. The latter part of the name derives either from the Irish tír or from the Old Norse staðr, both of which translate as "land" or "territory".
Úgaine Mór (Hugony the Great), who supposedly built the hill-fort of Dún Ailinne, near Kilcullen in County Kildare, united the tribes of Leinster. He is a likely, but uncertain candidate as the first historical king of Laigin (Leinster) in the 7th century BC. Circa 175/185 AD, following a period of civil wars in Ireland, the legendary Cathair Mor re-founded the kingdom of Laigin. The legendary Finn Mac Cool, or Fionn mac Cumhaill, reputedly built a stronghold at the Hill of Allen, on the edge of the Bog of Allen, in what was then Leinster.