Assuming you're on time (and immigration is quick), it's certainly enough time to go into the city and do one of the big sites - Guinness Storehouse, Kilmainham Gaol, etc.

For something less full on, Malahide is apparently nice, though I've never been.

Or you could just find an airport bar, have a meal, read a book, etc.! Depends on your energy levels I guess.