Well, starting in yr 11 would be quite problematic, if you just go straight in, half way through the course. However, if you did the whole course in just one year in half the subjects-perhaps ones you didn't study in Ireland-that would be a better option, but I don't know anywhere that would do that. There's also the issue of coursework, as Hodan said. And if you start in yr 11, when the class was in yr 10, they might have covered bits of the syllabus that might not have been on the Irish equivalent. So going straight into yr 11 would be a bit odd really, if you haven't done the first year.
Starting in year 10 would be a better idea, although you've already done GCSE equivalents. But the English syllabuses might be quite different to the Irish ones, so there might not be that much overlap. And if there is, you'll just have an advantage! And you're the age at which most people go into yr 10 anyway, so you'll be with people exactly your age. And also you could choose subjects that you didn't do in Ireland, if you wanted, to make it more interesting for you.
Starting in yr 12 would feel odd, I'd think. You'd be with people mostly 2 years older than yourself, so you'd feel quite young I'd think. And the A-Level course might build from stuff in the GCSE that wasn't in the Irish certificate, so there would be some completely new stuff that you're expected to already know. Then there'd be the issue, when the time comes, of going to university. I don't know if there's a minimum age you can go, but if you go into yr 12 now, you'll only be 16 when you start uni (unless you take a gap year), and hence you won't be able to do certain things that other students would be able to do for at least a year. However it'd mean that you finish education 2 years early and hence be at a degree level early so get a career early and stuff.