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.
Peerage of Ireland
The last two grants of Irish peerages were: the promotion of the Marquess of Abercorn (a peerage of Great Britain) to be Duke of Abercorn in the Irish Peerage when he became Lord-Lieutenant of Ireland in 1868 and the granting of the Curzon of Kedleston barony to George Curzon when he became Viceroy of India in 1898. Peers of Ireland have precedence below peers of England, Scotland, and Great Britain of the same rank, and above peers of the United Kingdom of the same rank; but Irish peers created after 1801 yield to United Kingdom peers of earlier creation. Accordingly, the Duke of Abercorn (the junior Duke in the Peerage of Ireland) ranks between the Duke of Sutherland and the Duke of Westminster (both dukes in the Peerage of the United Kingdom).