Ireland has made a significant contribution to world literature in both the English and Irish languages. Modern Irish fiction began with the publishing of the 1726 novel Gulliver's Travels by Jonathan Swift. Other writers of importance during the 18th century and their most notable works include Laurence Sterne with the publication of The Life and Opinions of Tristram Shandy, Gentleman and Oliver Goldsmith's The Vicar of Wakefield. Numerous Irish novelists emerged during the 19th century, including Maria Edgeworth, John Banim, Gerald Griffin, Charles Kickham, William Carleton, George Moore, and Somerville and Ross. Bram Stoker is best known as the author of the 1897 novel Dracula. James Joyce  published his most famous work Ulysses in 1922, which is an interpretation of the Odyssey set in Dublin. Edith Somerville continued writing after the death of her partner Martin Ross in 1915. Dublin's Annie M. P. Smithson was one of several authors catering for fans of romantic fiction in the 1920s and 1930s. After the Second World War, popular novels were published by, among others, Brian O'Nolan, who published as Flann O'Brien, Elizabeth Bowen, and Kate O'Brien. During the final decades of the 20th century, Edna O'Brien, John McGahern, Maeve Binchy, Joseph O'Connor, Roddy Doyle, Colm Tóibín, and John Banville came to the fore as novelists.

Answer this question based on the article: How many pen names did Brian O'Nolan have?
3