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 Peerage of Ireland consists of those titles of nobility created by the English monarchs in their capacity as Lord or King of Ireland, or later by monarchs of the United Kingdom of Great Britain and Ireland. The creation of such titles came to an end in the 19th century. The ranks of the Irish peerage are Duke, Marquess, Earl, Viscount and Baron. As of 2016, there were 135 titles in the Peerage of Ireland extant: two dukedoms, ten marquessates, 43 earldoms, 28 viscountcies, and 52 baronies. The Crown of the United Kingdom of Great Britain & Northern Ireland continues to exercise jurisdiction over the Peerage of Ireland, including those peers whose titles derive from places located in what is now the Republic of Ireland. Article 40.2 of the Irish Constitution forbids the state conferring titles of nobility and a citizen may not accept titles of nobility or honour except with the prior approval of the Government. As stated above, this issue does not arise in respect of the Peerage of Ireland, as no creations of titles in it have been made since the Constitution came into force.