How the heck did you get out of training?!!! I've rowed for all my years here (4 so far), been on committee for 3 and was Captaining last year, and did all (ok, most) of the training! didn't get to socialize that much cuz med stuff was really hectic last year, but I agree....the social side is very fun, a bit insane, and definitely...unique. I'm actually headed to the world championships in Eton next weekend for the 3-day party!!!
In dublin, both TCD and UCD have class boatclubs/equipment. If you want to try out rowing, this would be the time to do it because it'll only cost like 3 euro to join (as opposed to the 300-400 euro annual membership fee at other clubs). Rowing in both colleges is also one of the most prominent sports on campus. There are also a few rowers at RCSI. UCC has a rowing team (their novice men were successful this year), as does NUIG (quite big down there too).
It's not something I can really explain without you trying it. The life is fun, if not dramatic, and you can fall in love with it your first day. Not to mention get great fitness. You do spend a significant amount of time training, but that's partly social too!
Rugby's also fun. I played rugby as well my first two years of college. For the guys, there's the Hospital's Cup which is great for networking, and very big among the med faculty. I think the Hospital's Cup is one of the oldest "tournaments" in the country/world. College rugby is very hard to join without any experience though. I like it better than American football (blasphemy, I know) because I think it's faster-pace and more exciting.
I think medsoc has fun but I wouldn't really say it qualifies as the beersoc...they do very important and useful events besides socials/parties. IMSA is a separate organization.