agree with 2shortplanks. for a short visit, if you pick up the tour bus you get to see most of the sites around dublin, and halfway round it drops you off at the guiness stonehouse, and you pick it up again when you've finished. you can get off again at the arlington hotel which is a nice bar for some snacks, more guinness and some irish jigging. from there you can toddle down grafton street to window shop and have yourself a photo alongside the tart with the cart.
if you aint been to dublin before then you'll probably end up at temple bar for some nightlife. despite everyone bemonaing it being a tourist trap, its good fun/craic for visitors and after all, you are tourists! i always try to get a meal in at gallagher boxty house as well - its become a tradition for us as we knew someone who had an 'interest' in the place, and the food is very good.