Pitts S2B advice is right on the mark, and I'm wondering which aerobatic schools he is referring to
If you don't have a green card (and I don't know whether the Green Card Lottery is still weighted heavily in favour of Irish natives, if it still exists at all) you will not get a "real" flying job over here, ever.
Instructing is not bad at all, in fact you will probably learn as much as your students. Well, a whole day in the pattern can get boring, but right now that's not as hard as getting that instructing job in the US. Currently there are a lot of CFIs walking around US flight schools dropping off resumes/CVs, and they are mostly wasting their time.
Back in the good old days, it was possible for a foreign national to get a CFI position if they paid the flight school to earn it. So you drop (lets say) $20k-25k to get your FAA Commercial (includes IR), and CFI, and the INS would let you stay long enough to get your 1500 hours with 100 multi. Then you either find a US fiancee, or go home. Back in Europe you have loads of experience, but the regulators don't care, and you have to drop another xx grand to take your JAA CPL/IR quizzes all over again
Anyway, to answer your question "Is there a realistic chance of getting an FAA FI position with low hours"
I would say right now no. However if you come here with the right visa, drop the cash and get on with the owner I would say you would be first in line to get any job that comes up. My opinon would be that I'd hold off until the US airlines start hiring again, as right now the CFIs with jobs are not moving.
Just for the record, a "frozen ATPL" (which over here is equivalent to a Commercial certificate) will be accepted for issuance of a private licence, on the spot. In fact, if you turn up in the US with your JAA CPL (or frozen ATPL as you call it) I don't believe you will qualify for any scheme which allows you to instruct. You need to start from scratch - you are essentially buying the priviledge to work here as a CFI by paying for your CPL training. (Though I stand to be corrected on the latter, I would bet a lot of beer that it is correct advice).