Trinity College Dublin or UCD?

I went through the same dilemma as you (ended up getting my first preference course in UCD). They're both pretty similar, with a lot of cross-university socialisation, that is, lots of people are friends with people who go to the other institution. TCD is by far superior in terms of campus, because it's a landmark, but by far behind in terms of having a utilitarian campus. UCD is practically a town, with something like 3 gyms, Olympic pool, multiple residences, security, vet hospital, buildings ranging from business to graduate business to science to vet, to CS, to med, to law, to agricultural sciences and studies, etc. UCD has a rich history within Ireland, and (IMO) superior alumni. I remember one of my first lectures (history) where a picture was summoned on the projector with the leading political figures of Ireland's state formation, and my lecturer pointing out the many alumni of the very institution I was sitting in and a member of, e.g. Eoin Macneill (professor) and his wife, along many others. UCD was pretty central to the Irish Revolution and is the #1 in Ireland for history, particularly Irish history. TCD is the only competitor (in global history, although I would argue both departments are on par, but it cannot and does not compete for Irish history). Also, the structure of courses allowing for electives in anything every semester might be attractive for you. You cant go wrong with either, but dont choose TCD for its campus or its marketing, as you might end up disappointed. I know a few who transferred from TCD to UCD (and vice versa).