The Duke of Leinster (named after Leinster, pronounced "Linster") is Ireland's premier peer.

Initially called the Earls of Kildare, the family, called the FitzGeralds, came to Ireland with the Normans in the eleventh century, becoming as was said more Irish than the Irish themselves, a phrase which noted how various new waves of arrivals in Ireland adapted Irish culture, religious practices and the language of the native Irish, becoming exactly what the phrase implies. Two senior FitzGeralds, Garret Mór FitzGerald and his son, Garret Óg FitzGerald served as Lords Deputy of Ireland (the representative of the King of England in Ireland).

The subsidiary titles of the Duke of Leinster are: Marquess of Kildare (1761), Earl of Kildare (1316), Earl of Offaly (1747), Viscount Leinster, of Taplow in the County of Buckingham (1747), Baron Offaly (1620) and Baron Kildare, of Kildare in the County of Kildare (1870). The Viscounty of Leinster is in the Peerage of Great Britain, the Barony of Kildare in the Peerage of the United Kingdom, and all other titles in the Peerage of Ireland. The courtesy title of the eldest son and heir of the Duke of Leinster is Marquess of Kildare.

The family was originally based in a large castle in Maynooth in County Kildare. In later centuries the family owned estates in Waterford with country residence being a Georgian house called Carton House which had replaced the castle in County Kildare. In Dublin, the Earl built a large townhouse residence on the southside of Dublin called Kildare House. When the Earl was awarded a dukedom and became Duke of Leinster, the house was renamed Leinster House. One of its occupants was Lord Edward FitzGerald, who became an icon for Irish nationalism through his involvement with the Irish Rebellion of 1798, which ultimately cost him his life. Leinster House was sold by the Leinsters in the early nineteenth century. After nearly a century as the headquarters of the Royal Dublin Society, which held its famed Spring Show and Horse Show in its grounds. In 1922, Oireachtas Éireann, the two chamber parliament of the new Irish Free State, rented Leinster House to be its temporary parliament house. In 1924 it bought the building for parliamentary use. It has remained the parliament house of the Irish Free State (now called the Republic of Ireland).

The Dukes of Leinster had by the early 20th century lost all their property and wealth. Their Carton House seat was sold (though one of Ireland's most historic buildings with perfectly preserved 18th century grounds, it was controversially turned into a hotel and golf course in the late 1990s by the current owner in an act condemned by environmentalists), as later on was their other residence in Waterford.

Dukes of Leinster, First Creation (1691)

• Meinhardt Schomberg, 3rd Duke of Schomberg, 1st Duke of Leinster (1641-1719)

Dukes of Leinster, Second Creation (1766)

• James FitzGerald, 1st Duke of Leinster (1722-1773)
• William Robert FitzGerald, 2nd Duke of Leinster (1749-1804)
• Augustus Frederick FitzGerald, 3rd Duke of Leinster (1791-1874)
• Charles William FitzGerald, 4th Duke of Leinster (1819-1887)
• Gerald FitzGerald, 5th Duke of Leinster (1851-1893)
• Maurice FitzGerald, 6th Duke of Leinster (1887-1922)
• Edward FitzGerald, 7th Duke of Leinster (1892-1976)
• Gerald FitzGerald, 8th Duke of Leinster (1914-2004)
• Maurice FitzGerald, 9th Duke of Leinster (b. 1948)

Heir Presumptive: The Lord John FitzGerald (b. 1952) (The Duke's brother)