I attend UCD in the GEM programme and I recommend it to applicants on the fence.
I don't know much about RCSI (except that they take exams after Christmas break) so I can't really comment on their programme.
At UCD the majority of professors are good at their jobs and are approachable. There are only one or two horrendous ones that come to mind and they have a very small role in your education, which is a good thing. The programme has a fair structure and grading scheme (for the most part).
All of the facilities that you will you use as a medical student were built within the last 10 years (the lecture halls, the library, and the anatomy lab).
UCD has longer holidays than RCSI (winter and summer) as they follow the university's academic timetable which is good if you want to complete electives abroad, study for the USMLE, etc and UCD finishes their exams before going on Christmas break. Some students got 5 weeks off at Christmas, others got 4 (depending on which option they selected).
UCD costs less than RCSI for Irish students and international students.
Our first two years are almost entirely conducted at the Belfield Campus (in Dublin) with the exception of GP placements. The last two years we are split between St Vincent's and The Mater (both in Dublin). I'm not really sure where RCSI students go for their rotations as I think St James's is Trinity's Hospital. Maybe an RCSI student can shed some light on that.
Feel free to ask further questions.