In the book Harry supports Ireland, but prefers the Bulgarian Seeker, Viktor Krum. In the movie, he supports Bulgaria.
He supports both. A true sportsman, he enjoys the sport itself, and not the competition and rivalry of it (except when it comes to beating Slytherin). Ireland is as close to a home team, and has superb chasers that are a joy to watch, and Bulgaria has the world's best seeker, a position Harry is very interested in because he's a seeker himself. In the book, he considered Krum the most exciting player in the pitch.