In Dublin, there are lots of pubs but unfortunately not much choice in what beer to drink.
There are a couple of exceptions: 1) The Bull and Castle pub in Christchurch is the pick of the bunch, has good food, and a good selection of Irish craft beer on draft and in bottle. They always have an Irish craft beer on cask, usually O'Hara's stout (much better than Guinness) or Goodstore IPA.
2) The Porterhouse in Temple Bar has several of their own beers on draft, live music in the evenings and generally has a good atmosphere. http://www.porterhousebrewco.com/bars.html
The above two pubs will probably be more enjoyable and less crazy if you don't visit them on Paddy's day.
I don't know much about beer related stuff in Belfast although I think the Hilden brewery is worth visiting. http://www.hildenbrewery.co.uk/
The Irish Craft Brewer website has a listing of pubs serving craft beer here: http://www.irishcraftbrewer.com/inde...d=91&Itemid=69
The site also has a forum where people will be happy to point you in the right direction for beer related activities.