My methodology has led to more than the usual amount of homework here. A lot of the books that are tagged "Northern Ireland" are not in fact all that popular, and I had to do some serious drilling to get the results by number of owners. Once I had done that, both LibraryThing and GoodReads threw up a number of false positives with books whose authors are from Northern Ireland (C.S. Lewis, Flann O'Brien, Maggie O'Farrell) or which discuss Northern Ireland without being set there (Trinity, gawd help us, by Leon Uris).
But I have ended up with two light novels, neither of which I have read but both of which I may give a try. The LibraryThing winner, first published in 2004, got re-released in 2007 and has been a huge hit in Canada (whose profound links with my homeland are not always appreciated). It is set in a thinly disguised Holywood, County Down, before the Troubles, and is the first in a series of ten books. It is:
An Irish Country Doctor, by Patrick Taylor.
From County Down we move to a fictional village in County Antrim ("the armpit of Antrim, on the north of the north coast of the north of Northern Ireland") for the GoodReads winner, also the first in a series, also published in the mid-2000's (2006 to be precise), a supposedly humorous mystery story about a Jewish immigrant to Ulster who is accused unjustly of Crime, which is also the book most often tagged "Northern Ireland" on both Goodreads and LibraryThing and therefore I suppose is today's winner:
The Case of the Missing Books, by Ian Sansom
I find it interesting that both of them skirt the Troubles chronologically, one set before and the other after.
On LibraryThing, I may have to disqualify Transatlantic by Colum McCann because I think only a section of it is set in Northern Ireland. Next after that, and far ahead of the rest, is a childhood memoir and a Booker nominee, Reading in the Dark by Seamus Deane. The second-ranked book on GoodReads is a Troubles thriller, The Ghosts of Belfast by Stuart Neville which I had not heard of.
The top non-fiction book on both systems is The IRA by Tim Pat Coogan.
Colin Bateman, Robert McLiam Wilson and Seamus Heaney are not too far down the list.
(After this I'm going to run through sovereign European states, in descending order of population - starting with Russia.)