Try the Guiness Brewery, this is one of those things you just do when you come to Dublin. I read many reviews talking about what a disappointment it was, but I have to disagree. I was pleasantly surprised at the museum. It is a self guided tour of a display they have set up to show the process of making it. Cost is 15 euros for any adult.

Dan
http://www.pricelesstravel.co.uk
BuzzWorld

I quite liked Dublin Castle, its in a central location in Dublin and offers a good photo opportunity. The guiness at the brewery is also excellent tho!

Niall
http://www.roomsindublin.com