Well there's lots of reasons I like RCSI, but to answer your question more directly, I guess one of the reasons I chose it over the other schools is the fact that RCSI actually cares enough to send people over to interview you. As a by-product of the interview, I was forced to research the school more carefully and noticed a lot of things I like about that school (one being the emphasis the school places on early clinical exposure). Moreover, to my knowledge, RCSI was the first to institute a graduate entry medical program... so they would have had the opportunity to get more feedback from actual students about what works and what doesn't.
I also know two people there (1 graduate who started his residency in Internal Med at McMaster University; the other going into 4th year) and both said that RCSI has people dedicated to helping you get a placement for residency back in North America.
Curious as to why you guys would rather go to UCD? Is it just the difference in tuition?