For an FAA PPL you don't need to do a formal groundschool. You can just get the books and study on your own (or get the King videos or something like that). Do your studying before you go to the US to fly, take your written there, and you're set for your flying where the experience will start to make more sense of what you learned in the books.
The FAA written is much easier than the whole JAA malarkey. One test, can't remember how many questions, takes less than an hour if I remember correctly. But don't forget it because your flight examiner may pull some questions on you as well. Someone fill in for me here - is there an oral part to the flight exam? It's been 20 years so I can't remember. I can tell you what kind of hat the guy wore, though!
The challenging part for you in Ireland will be getting FAA medicals and biennials, though they may be more readily available than I'm thinking. I've never bothered to look, even when I was flying in the UK.
Also, if you haven't yet, check into any international limitations an FAA license may have in JAA land. For instance, when I was doing it all back in '98, I seem to remember that I could fly G-reg aircraft in the UK on my FAA ticket, but I could not fly a G-reg outside of UK airspace. If I had a N-reg airplane, no problem anywhere. Might be different now. I'm not sure. If it's the same, it still gives you lots of scope for even domestic flying, which most of mine was anyway.
Hope some of that is at least relevant, if not exactly helpful.