I'm not 100% sure, but if you got an S3 in the US that's SIM-free and unlocked, you'd probably be buying the same international version as in Ireland anyway. As far as I know, that's the only model available unlocked anywhere. The rest are all tied to their respective carriers. That's just how it works in the US.
That said, it may be possible to unlock an S3 from AT&T or T-Mobile to use in Ireland (maybe even the Verizon version, but I can't guarantee that). It just seems like a waste of time to me when you could just buy the international version unlocked - I mean, unless you really, really want the extra gigabyte of RAM and don't care about the Exynos processor.