I lived in Dublin for a few years and the boating is great. I don't know what water levels are like right now. It pretty much has to be raining to boat. If I was heading out to surf I would head to a town on west coast called Lahinch. At least when I lived in Ireland they tended to have good surf there. There's also a great pub that I can't remember the name of, but they don't care if you're pretty manky from boating and it's a great place to warm up. The shop that I remember is called the Great Outdoors Head up Grafton Street towards Steven's Green when you get to the top of the street (it's a pedestrian mall) The shop is on a side street to your right if you're facing the green (and it's still there after 7 or years.) The college I went to is at the bottom of the street (trinity) They're kayaking club's web page is :http://www.tcd.ie/Clubs/Canoe/
you might be able to get hold of someone to meet up with to paddle. Also, you can search for the Irish Canoe Union web page which might be of some help. BTW, they call kayaks Canoes, so don't be confused. There is great surfing in Kerry as well (SW part of the country.) But Lahinch is great fun. IF you're going there, stop and run Ennistimon Falls... ( I don't remember how to spell it, but it's in a town on the way and about 400 yards of super fun in the middle of a town.) If you're really interested, I think I still have the Irish whitewater guidebook somewhere.
Good luck and have a great time,