As with Wales, and even more crushingly, there is a very clear winner in this category. Most frequently tagged "Ireland" on both GoodReads and LibraryThing, and owned by more users on both systems than any other book set in Ireland, it won a Pulitzer and dominated the best-seller lists of 1997. It is, for once, a work of non-fiction.
Angela's Ashes, by Frank McCourt.
There are in fact two books even more popular among GoodReads users which have been tagged "Ireland" because of the origin of their authors, but as far as I remember (and I've re-read both fairly recently) no part of Dracula or The Picture of Dorian Grey is set in the writers' homeland.
On Librarything, it's fairly close at the top, though, with the winner just a nose ahead of Ulysses, A Portrait of the Artist as a Young Man and Dubliners in terms of number of readers.
Those three are a bit further down the list on GoodReads, though with P.S. I Love You by Cecelia Ahern and In the Woods by Tana French in second and third place, and Darkfever by Karen Marie Moning in fifth (after the adaptation of Homer, but ahead of the student memoir and the short stories). Is it worth my checking any of those out?
Bubbling under: Roddy Doyle, Maeve Binchy, Leon Uris (gawd help us),How the Irish Saved Civilization by Thomas Cahill.