any suggestions on the must see places in dublin in one day?

Guinness Storehouse.
http://tinyurl.com/dgb8dr

My suggestion would be the hop on-hop off bus. That way you'll get a good overview of the city and can stop anywhere along the way that interests you. Here's a link for one of the two companies that do them:

http://www.dublinsightseeing.ie/cityTour.aspx

I agree with the hop on/hop off suggestion. It is a great way to see what you want to see.

Things I've liked in Dublin:
St. Patrick's Cathedral
Dublinia
Guinness Factory
Jameson's Whisky Distillery
Kilmainham Gaol
Trinity College/Book of Kells/Old Library
Grafton Street Shopping
Molly Malone Statue
O'Connell Street for people watching
Brazen Head Pub (oldest pub in Dublin)

IMHO the two most important are the Book of Kells at Triity and Dublin Castle. I wouldn't do a bus - the center of Dublin is VERY small and yuo can esily walk anywhere you want to go in a few minutes.

Be sure to check out St. Stephen's Green and take a stroll on Grafton Street and some of the smaller streets nearby. Try to take in some of the architecture of Georgian Dublin around Merrion Square...this is the nicest part of Dublin. And of course, save time for a pub!

At the top of my list are Trinity College and the Guinness Factory. You should definitely walk along O'Connell Street.

Buffbeauty - that's the name of one of my favorite roses. What a great screen name!