Hi,
I'm a final year medical student at RCSI, from Canada. Don't worry too much about the interview. I would think that it's just a chance for the interviewers to see that you are 'NORMAL', ya, that's pretty much what they're looking for. Sorry the 'secret' wasn't juicier. They judge you on your 'paper application' merits, the interview is more or less meant to weed out the 'crazies'. However, it doesn't always work out that way. The interview probably won't rocket an applicant into contention if they weren't there in the first place, but if you're weird or have an antisocial personality disorder, than the interview can crush your chances. Best advice is to be honest, respectful, and humble, be prepared to talk about anything that you included in your application, and find out as much info on RCSI, the financial implications of attending, and what you need to do in order to license back in the US or Canada when you finish. Don't waste your time thinking about staying to practice in Europe after, you probably won't want to, just trust me on this one. And even if you do, that's easy to do once you're studying in Europe.
Interview is about 30 minutes (at least when I went), usually with 2 or 3 interviewers. Interviewers are usually people like the Dean, the Vice-Dean, the Examinations Officer, the Registrar, some professors, etc.
Beforehand, you meet one of the guys from the ABP and sit in a room with other interviewees. The interview is pretty relaxed, but I've heard people have been asked weird questions, that's probably because it's been a long day for the interviewers and they're now getting bored.