Inspired by the RCSI student thread. I can answer your questions.
1) A typical day for a UCD med student is vary laid back. You will have maximum 5 hours of lectures a day. Some days are longer with dissections or tutorials, but those are the minority. Some students prefer not to go to lectures and to study independently; how you learn best is up to you. Most learning is didactic in pre-clinical years.
2) When I went through it, no help was given to students to study for the USMLE. Now I hear they are trying to start a USMLE prep course similar to RCSI. They wanted to do one this year that students could take as an elective class an get credits for, but it fell through at the last minute. Hopefully it will be back on track for next year.
They will pay for you to take a NBME self-assessment exam, and pay for you to reschedule your USMLE if you fail your NBME. This is because too many (mostly Irish) students were failing the USMLE, and it made UCD look bad.
3) The match rate for UCD students back to USA is about 60%. It is low, but fairly on par with most of the Irish medical schools. This is why I would not recommend studying in Ireland unless you have EU citizenship, so you can complete internship in Ireland as a backup.
4) For the most part, students have to set up electives on their own. There are about 10 that are organized through the university and are funded with scholarships, but these seem to be given to mostly Irish students. It is fairly easy to organize electives, though, and older students are helpful with places that are happy to take UCD students.
5) There is not a strong alumni network. The GEM program is fairly new, though, so maybe that is why. Your best bet would be making friends with an older year and asking them for advice. There is no organized system. However, in my personal experience, most alumni will answer questions through email.
6) The 60% match rate is everyone, Canadians and Americans. I didn't include Irish because the Irish application rate to the US is very low, especially right after final year. I think there was one person in the last 4 years (they matched).
Typically Americans do better applying to the US because they do not require a work visa. This means less work for programs and no chance of you having potential visa difficulties that could prevent you starting on time. I don't know the actual breakdown between Canadian vs American applying to the US, though.
7) If I had to do it all again, I probably would, especially because I matched. If I didn't match than I would have 100% regretted it (...obviously, I suppose).
The main reason that I don't regret it is that being a doctor is the only thing I've ever wanted to do. If you can see yourself something something else, do that. You have to REALLY want it, because there's going to be a lot of stress and a lot of hard work along the way.
8) There is stigma about being an Irish medial graduate. Mostly a few comments here and there about how you couldn't get in to medical school at home.
But, in Canada anyway (sorry I can't speak about USA), there's no stigma career-wise. I've met Irish medical grads that became Chief residents, and that got great fellowships in many different specialties. Once you're in, it's all about how you perform as a resident. Nobody except for very elitist people cares about where you went to medical school 5, 10, 20 years down the line.
And there is WAY less stigma than being a Caribbean grad. The Irish medical schools are excellent, and I actually feel like I received a better clinical education at UCD than I would have in Canada. And it is well known, especially among residency selection committees, that Irish medical schools are more difficult to get in to.