Actually, the more I think about this, the more I think most Irish speakers would just use the word "simpatico" and likely wouldn't use an Irish word. As others have pointed out, there is no English word which is equivalent. I would guess the same is true for Irish. When that happens, the Irish, just like English speakers do, use the word itself, which doesn't translate. Nowadays, you hear Irish speakers using many contemporary usage words that aren't Irish, but may be English, French, etc. and have crept into usage in the same ways those words have in other languages.