I believe UCD is the most prestigious of that lot, but I doubt that any of them are going to hurt you compared to the others when it comes time for the Match. The admissions difficulty/acceptance questions would probably be best posed to the good people at Atlantic Bridge: US and Canadian students studying medicine, dentistry, and veterinary medicine in Ireland, who process the applications.
As far as locations, it really depends on what you're looking for. It would seem that UCD and RCSI are located in areas with the most going on, but Ireland it beautiful... plus, it's medical school. You'll be kept plenty busy with studies. Good luck!
"When I haven't any blue... I use red."
- Pablo Picasso