I went to Dublin for I&E and it was great! Dublin is a nice compact city good for walking and plenty of cab or bus options. Definitely stay away (do not get a room!) from Temple Bar, overpriced pubs and too touristy. We stayed on the North side, Castle Hotel, which is really nice too. Lots of shopping and of course great pubs all around. There's all the usual touristy things to do like Guinness factory and the like. Try the 'little Museum Dublin', they have a section that is all U2 related. There is also a rock and roll museum of Ireland, not far from Temple Bar.
If you want a private fan tour of u2 related sites, check out Dave Griffith's tours, he is a friendly great guide around the city for about $30 or so.
The 3Arena where the show most likely takes place is smallish, about 12k people fit in there. Its easily accessible by the local rail.
Most of all HAVE FUN! It is a blast to be where U2 is from!