Embryology, histology, genetics and molecular biology / immunology. This is where it gets difficult because you're trying to teach yourself some pretty difficult subjects. Also, you'll realize that stuff you thought you knew well (ie. physiology) are not asked in the same way as you're used to, so you nearly end up having to learn it again to meet the US system. Really bone up on pathology as well (which might be better taught than I had it). Detailed microbiology (there are some regional bugs common to the US which obviously aren't taught overseas).
I saw some report somewhere that showed by subject how USMLE takers from different countries compared to US students. I tried to find it but I've either deleted it or lost it. Sorry!
Anyway, the good news is that students from Ireland did just fine in pathology, physiology, and pharmacology. The two subjects we got killed in were Biochemistry and Behavioural Science.
I can personally say that in my practice tests I was doing REALLY badly in those two subjects and molecular biology as well.
I'm not sure how UCD compares to UCC but for our micro we didn't get a lot of emphasis on lab tests. I was doing great on "clinical" micro questions but not so hot on the laboratory type questions. I think I bought BRS microbiology (in addition to Micro made ridiculously simple) and BRS molecular biology. Both have no pictures - just straight text - but they helped me a lot.
Also remember, just because Behavioural Science is fluff doesn't mean it's easy. Well, it isn't hard or anything but I think it took me a lot of practice. I found the worst thing about BH was that often all 5 of the possible answers on the real test sounded WRONG where as a lot of the commercially prepared USMLE questions for BH had 5 answers of different shades of RIGHT. There often wasn't a great option on the real test and sometimes all of the answers sounded at least mildly inappropriate and I had to pick the least worst one. I found some of the better question banks really did a better job of preparing me for the BH questions then some of the review books did.
I thought Behavioral Science is the one subject that wasn't covered well in the review books. The best way to study for it is just practicing the questions with good question books or the qbanks. I used both Kaplan qbank and USMLEWorld during 3rd year and thought they were both good.
The review books I had (High Yield BH and something else) had way easier questions I thought. Also, they covered more BH "topics" where as all the questions were kinda situational based. Like I said before, I often found that on the real test NONE of the answers sounded right or even appropriate sometimes.
Anyway, BH is actually really easy if you get any resource really and just do a bit of practice in it. I think the problem is most people ignore it because it seems so soft and fluffy. Especially since a lot of the BH in review books is like psychology for idiots. Don't worry about it just don't ignore it =).