Royal dukedoms in the United Kingdom
In the British peerage, a royal duke is a member of the British royal family, entitled to the titular dignity of prince and the style of "His Royal Highness" who holds a dukedom. Dukedoms are the highest titles in the British roll of peerage and the holders of these particular dukedoms are Princes of the Blood Royal. The holders of the dukedoms are royal not the titles themselves. They are titles created and bestowed on legitimate sons and male-line grandsons of the British monarch, usually upon reaching their majority or marriage. The titles can be inherited but cease to be called "royal" once they pass beyond the grandsons of a monarch. As with any peerage, once the title becomes extinct, it may subsequently be recreated by the reigning monarch at any time.
Duke of Leinster
Duke of Leinster (; ) is a title in the Peerage of Ireland and the premier dukedom in that peerage. The subsidiary titles of the Duke of Leinster are: Marquess of Kildare (1761), Earl of Kildare (1316), Earl of Offaly (1761), 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".