GF studied abroad there in 09. Here are her standard recs:
The Guinness Factory/the Jameson Distillery are definitely the touristy must-do things -- I like the distillery a lot better, because there is actually a guided tour and Jameson is delicious, but the Guinness Factory (which is a self-guided tour, or at least was five years ago) is worth it for your "free" (with the price of admission) pint at the sky bar, which has great views. Other things of that sightseeing nature include Trinity College, which is just pretty to walk around, and Grafton Street, which is where a lot of the shops/street performers/crazy city sights are. It leads you right to St. Stephen's Green which is a pretty Dublin park/another sightseeing destination.
Some of my favorite bars included Messrs, which was this four-story bar that in 2009 had a house brew that was 4 euro a pint (which, sadly, was cheap), though I can't confirm the price point now. It's also a great place to watch games; the whole place goes nuts for rugby and footie. Temple Bar is sort of the obligatory pub area, and therefore gets very crowded and a little touristy, but Temple Bar in Temple Bar is a cool spot, as is Fitzsimon's which always had a high cover but was worth it for the rooftop. Fitzsimon's is also one of the places in Dublin that's open after 2 a.m. (when most other pubs close down) so, if that's something you're interested in, that's an option. You can also try McDonaghues, which is a crazy old pub that always has live, traditional music.
I know you're only there for a short time, but some cool day trips include Glendalough, which is great for hiking. You can also hop on the DART for a quick trip to the cool little villages outside of Dublin. One personal fave of mine was Howth, which was a fishing village with great views, hiking, and a cool farmer's market if you're there on the weekend.
I would recommend more food places but obviously none of the food in Ireland is outstanding. Your safest bet is some soup and brown bread at a pub. When I was there I did like Lemon (a crepe restaurant on Dawson, which runs parallel to Grafton), and Cornucopia, a vegetarian restaurant also near Grafton. Both were pretty casual, though.