What was the birth name of the protagonist in Barry Lyndon?

The birth name of the protagonist in Barry Lyndon is Redmond Barry.

The character begins the film as Redmond Barry, a young Irishman from a genteel but impoverished Protestant family. After his father dies in a duel, Barry becomes involved with his cousin Nora Brady and flees Ireland after a staged duel forces him to believe he has killed her suitor. His adventures then carry him through the British and Prussian armies, gambling circles, and European high society.

Barry eventually marries the wealthy Lady Lyndon in 1773. Through that marriage, he takes her surname and becomes known as Barry Lyndon, although he never fully achieves the aristocratic status he wants. The change of name therefore marks his social ambition rather than a different identity from the start of the story.

The film adapts William Makepeace Thackeray's 1844 novel The Luck of Barry Lyndon. The title character's original name is easy to confuse with his later assumed style and title, but “Barry Lyndon” is not his birth name. Ryan O'Neal plays Redmond Barry, later Redmond Barry Lyndon, in Kubrick's 1975 film.