Sure
Well IR's by far the most popular course at St Andrews, so popular in fact that this year they've had to move out of lecture rooms and into the music centre because there just wasn't enough seats for everyone in every lecture! Because of this I think they try to make the first two or three lectures quite boring so that the undedicated drop it and they have more manageable numbers :P It does get better, I promise!
First year you have a different lecturer for every week, so there's quite a variety of personalities and teaching styles going on. Your tutor is the one you really get to know, and for first and second year your tutors are likely to be PhD students or recently qualified lecturers, which is quite nice
It is definitely quite challenging, but that's to be expected of course! First year is a little more theory based, and they've changed the format of second year around since I took it so I can't comment on that, but IR covers such a broad range of topics that you can expect to study lots of stuff you're interested in, and quite a bit of stuff you're not so interested in too. Third year, where you get to pick and choose your specialities, is much much more fun- I study joint IR and Russian so last semester I did a course on Russian Politics and Foreign Policy, which I found super interesting, and I'm just as excited for this semester's module as well
As for demographics... unsurprisingly, IR does attract a lot of international students. Many of these are American, but there are people from all over the world, really. In my last tutor group there were around five Americans, a couple of English people, a couple of Scottish, one Norwegian, one Georgian, and a Pole. There's such a mix of backgrounds that it doesn't really matter where you come from, and I wouldn't say anyone is discriminated against on the basis of their background or anything like that.
Hope that was helpful, I've tried to be as honest as possible... some of the theoretical stuff you will study in first year is a little dry, but if you stick with it it does become genuinely interesting. I read articles on my lecture topics purely for interest now, but maybe that's because I'm a bit of a geek :P