Congrats on your offers, Riverpeth! I did a year of my undergraduate degree at UCC and really loved it there; it's actually the reason I chose to apply back to Ireland for med school instead of staying within the States. That said, I'm pretty sure accepting my offer at RCSI for the fall. Imo, there are a couple of things you have to ask yourself:
Are you afraid of "the bubble"? At RCSI, you won't have easy access to non-medical folks, and meeting people as a newbie foreigner can be tough. If you're someone who values educational diversity in their friend group, you may prefer a university environment, where you'll get the chance to meet and greet with Arts students, as well as future doctors. If you're at a place in your life where you really just want to sink deep into the medical school lifestyle, the college may be a better fit. Also, RCSI separates out GEM and 6 year students for the first two years, which means no 17 year olds. Personally, I'm into that, but you may prefer a broader age range.
UCC will also offer a broader range of clubs and socs, while the ones at RCSI will be more medicine-specific. If you are desperately hoping to join a, idk, parasailing club in the next four years, a university is more likely offer that.
Dublin vs. Cork. At the end of the day, Dublin is a city and Cork is, honestly, a town. Don't get me wrong, Cork is the second biggest city in Ireland, and frankly, it's charming as hell. Very classicly Irish, imo. But fwiw, I had the city centre mentally mapped out within a week; by the end of the year, I felt like I'd been into every shop and walked the greater county. Of course, I'm a big big city kind of guy, so. Y'know. Cork isn't cosmopolitan, but it's lovely and tight-knit and it certainly has its own culture. Up to you, what you prefer!
How worried are you about matching back, etc? From what I've gathered, I think the point for USMLE prep (& the Canadian eq., probably, altho my research has been US-centric) has to go to RCSI. That said, in 2012, UCC had an impressive 94% USMLE stage 1 pass rate! So. I'd also compare curriculums. I like the anatomical focus at RCSI, and I like the rotations they offer. I haven't specifically compared to UCC, though -- that's something you may want to check on!
Cost, unfortunately, only you can decide.
Good luck! And congrats again.