If you've looked at some of the other threads, you will see that it is a bit competitive to get in. I don't really know on a scale compared with some US schools the nature of its competitiveness, but let's just say it's not a guaranteed shot. Each school has a quota of how many north americans they will accept, but that doesn't mean they fill that quota. For instance, I think Trinity is supposed to have spots for 25 North Americans, but my class has maybe about 15. Atlantic Bridge says the chances are 1:10 so for every 10 students who apply, 1 gets in.
About grading - maybe someone like Trinners can give you a clear view because even I'm unsure. If you are a direct entrant, you enter into 2nd year. It seems that while you receive marks for your 2nd year, 2nd and 3rd year marks may be combined. Some exams are cummulative - like 3rd year physiology exam includes material from 2nd year. So, it could be a meshing of "preclinical" material.
not all programs are 5 years. Trinity now is, I think UCD has a 5 yr. program as well. RCSI is 6 years, unless you're a direct entrant, then it's 5. And I'm not sure about UCC.
It would be possible to return to NY, but you should check your state administration to find out the details about competitiveness and eligibility for jobs. There's tons of Irish in NY 🙂 anyway.
