Have a look at this site: rollonfriday.com, in the discussion forum people ask this question about New York Bar often. It's mostly populated by English solicitors in big corporate firms. Their general opinion seems to be it's not much help to them, so I'd imagine it's the same over here. The AITI tax exams are something people in big firms seem to do, there would be a demand for solicitors who are also qualified tax advisors, but they're tough exams and you'd want to like tax...
European law in Ireland is mostly about competition law and regulatory stuff. Competition work is connected to Mergers and Acquisitions which can be insanely long hours when there is a deal on. The regulatory/compliance stuff could be pretty dry. If you want to find out what it's like to work in, try finding someone doing it who'll give you a few minutes to talk to you about it.
Only the bigger firms do European law and if you've applied to them and not gotten through, they would have your application on file and whether or not a subsequent application would succeed would depend I think on the reason they didn't give an interview etc in the first place and also what you did in the meantime. If you're committed to getting into that area, try doing paralegal type work in Luxembourg or Brussels firms, although you might need French...
The Comm LLM in UCD is pretty popular, but whether or not it will get you into the big firms is debatable. Do bear in mind that if you then want to get into a smaller/medium firm that doesn't do commercial work, they often have an attitude that they're wary of "academic" applicants as they're worried they aren't motivated to do the general practice job and/or could be know-it-alls.