Yes. I've never even met anyone who transferred from a Caribbean school to a US school, though I know it happens very occasionally...probably more common to transfer to a DO school or one of the larger US MD schools, if a spot opens up. You should assume your chances of being able to do this are vanishingly small...like <1% I would assume. You should therefore exhaust all reasonable avenues to get into a US schools, and then consider Irish/Australian/caribbean AFTER that. If you are reasonably young, like 20's, and this was only your 1st shot at US schools and you got even 1 interview, or your application was borderline to be able to get into a US school, you should seriously consider trying again even for at least 1 more year before you bail for Caribbean. It's going to take you 7-11 years of training after you enter/start med school to finish your training, so waiting 1 extra year to attend a US school would be more than worth it in the pain and expense and aggravation you would save later...not to mention better chance of being able to do a specialty that you prefer versus maybe being forced into one of a handful of specialties (primary care, psych, peds, etc.) even if you decide you don't like it.