The Kilkeel title was actually created especially for Prince Harry when he married, and there has never been a noble title connected with Kilkeel.
His Dumbarton title has been bestowed by a monarch on one previous occasion to George Douglas, a younger son of the first Marquess of Douglas, who was created Earl of Dumbarton in 1675.
Announcing the new titles, Buckingham Palace said: “The Queen has today been pleased to confer a Dukedom on Prince Henry of Wales.
“His titles will be Duke of Sussex, Earl of Dumbarton and Baron Kilkeel.
“Prince Harry thus becomes His Royal Highness The Duke of Sussex, and Ms Meghan Markle on marriage will become Her Royal Highness The Duchess of Sussex.”