I don't know if I should even be attempting to answer this question since I'm a teetotaler and not into the pub scene at all however I'll recommend one or two.
The Brazen Head, Irelands oldest pub dating back to 1198, has a good reputation.
Further back up river on the other side is Knightbridge in the Arlington Hotel close to O'Connell Bridge. I'm more recommending this because it comes with Irish song & dance entertainment.
Back across the river on the other side of the bridge is Messrs Maguires, a few minutes walk. It has a real Dublin feel to the pub, can get some good food there too.
Please view these links for better information:
http://www.pubguidedublin.com/publist/p1.htm about 9 years ago