Hey there!...This seems to be a tough choice that you have to make, but if i were you, i would also make a list of the pros and cons of St Andrews University, and then compare...But if i were in your place, i would personally choose Trinity over St Andrews, first of all, since Trinity is going to offer a 5-year programme, whilst St Andrews' course is a 6-year programme and i have a friend there in St Andrews who, although generally happy, is quite complaining about the fact that lecturers tend to give students 'more than enough' which you would not normally need in your clinicals, and most importantly, when you're practising. Nevertheless, Trinity has one of the largest and extensive libraries in Ireland, and probably in Europe!...Trinity, and generally Dublin, do also provide the necessary facilities and resources that you would need to undertake a successive path towards your studies, and i wouldn't say that you do have those opportunities if you were in St Andrews since, considering the fact that i've been there to visit my friend, i kinda found it to be a very 'limited city'!..And by saying this I'm not trying to diss any St Andrews students out there, it's just a matter of personal preference!...The other point i'm going to make is also about what i would prefer, and that would be different ways of approaching medical facts and knowledge. Trinity's Medical course consists of lectures, practicals, Problem-based sessions, small-group tutorials as well as case studies. This means that one would be able to develop his/her communication skills, critical thinking, and also team work skills. I'm not sure how it is exactly done in St Andrews, but i was told that one thing to note is that the PBL done in St Andrews is totally different than the one used in Manchester University.
Anyhow, that is one of the major decisions that you will have to make in your life, thus, i would recommend you to carefully consider the consequences of your decision, as i myself had to make a similar decision and am happy with where i ended up!
I'm sorry for posting such a long post!
Good luck with your decision!