Is it? I'm Irish and our cultures are quite intertwined between various media and I've never heard that before. I'll defer to you on it though! In any event, I reckon "swimsuit" should still be accepted as a more universally used translation, even if I wouldn't necessarily say it myself.
Yep, we really do say "swimming costume", or even "cozzie" for short. Not really sure how to spell that, it's very much a spoken-English thing. I think it's a bit old-fashioned now, though, and most English people would say "swimsuit". Interestingly that's what I typed, and Duo thought it should be "swim suit" with a space, but you guys seem to agree on the one-word spelling.