I didn't do an interview because Irish undergrads get in through the CAO which is completely points based however this is what I've heard from my friends who've interviewed.
Why do you want to do medicine?
Probably the hardest to answer I know I want to so medicine but I couldn't tell you exactly why it's a combination of so many things. 😛
Why Ireland?/Why RCSI?
Again hard to answer most people tended to go with the learning experience / experiencing different cultures etc.
It really depends on who you get interviewing apparently some of them have a conversation with you and you come out of it saying was that really an interview while others are very formal. They tend to discuss your past college experiences and your extra-curriculars.
Then the infamous How do you plan on paying for your education? or some variation on that theme.
Also have a bit of prep done about recent medical stories stories about Ireland etc from the newspapers one of our lectures was expressing his amazment at the no of candidates he interviewed who couldn't have a conversation about current affairs.
As I didn't interview and am going off second hand info I'll defer to those who did.
Good luck with the interview