What would you do with one day in Dublin?
Take the hop on-hop off bus tour and be sure and see Kilmainham Jail. The Dublin Bus,green and yellow tour buses, is the best one. Although I am in the minority I thought the Guinness Storehouse tour was very underwhelming.
I would definitely skip the Guinness Storehouse, too - way too expensive for a "free" pint, particularly since you don't see the brewing process. In addition to the hop-on, hop-off bus I would suggest walking through Trinity College campus and down Grafton St. There are almost always musicians playing along the street. Or walk down to Merrion Square to see the Georgian houses. The National Museum is nearby, too.
The places Nancy suggested are all on the bus tour so you will have no trouble finding them. There are several National Museums but I particularly enjoyed the one on Kildare Street. It is only a block or two from Grafton Street.
The Book of Kells and the other manuscripts were the highlight for me. Of course I have a thing for old manuscripts. I loved everything about the library.
Hop On/Hop Off bus will take you to all the major sites. It makes it easy to get around.
we took the Trinity College tour with a student guide and it ends at the Book of Kells...the view from the top of the Guinness Brewery is great. the tour not so much...the Guinness was the best though. You could skip to the top and drink the pint they give you...you can walk to St. Patrick's & Christ Church...Grafton Street. You can see quite a bit, Dublin is pretty small.
There are two editions of the "36 hours in..." series on the NYTimes for Dublin:
the other is here: http://travel.nytimes.com/2007/04/22/travel/22hours.html
I consulted them for ideas on my recent trip and I think they helped. :D