I hope you have a great time! Ireland is a beautiful place! I lived there in 2001, and loved it. There have been a lot of great suggestions already.
As far as Dublin goes, here are some suggestions:
Kilmainham Jail is very cool if you're into history.
St. Stephen's Green is beautiful, and a great place to sit and people watch while you have your lunch or something, providing it's a decent day weather-wise.
Grafton Street is fun, lots of shops and people.
The General Post office is cool too.
The Temple Bar district has some great pubs, and the atmosphere is very lively!
I ate at the Bad Ass Cafe (not sure if it's still there) a couple times, and it had great food.
I have several great guidebooks for Ireland, and can give you more info if you'd like. Send me a PM or an email
thora_seb@yahoo.ca and I'd be happy to help out!