Where are you now? If you are location flexible then I’d base the decision on which company feels best for you from the contact you’ve had and what you have read or heard that aligns with what you want from an employer. You also have the choice of big 4 and GT just outside the big 4 there too so there are the pros and cons relating to that. All in all though, they are all big accounting firms which will offer you a competitive contract (if you’re successful of course) and top notch training and experience, and the chance to work with intelligent ambitious individuals, so really it comes down to what you are looking for personally.
As for the cities, again it really depends what you want. I imagine you’ll probably have more to do in Dublin in terms of nightlife and shopping and that, but a quieter and more relaxed experience in the other two, with London just a bus away if you want it. Also, depending on where you are now think about the logistics of getting home to see family and friends (if that’s a concern). I’m not really flexible on location so I just applied locally and chose the firm I felt most suited me.
Hope that helps