"Is everything impossible?"
Translation:An bhfuil gach rud dodhéanta?
Your native Irish speaking friends are speaking a minority dialect (they are all "minority dialects") and they would consider the Irish spoken by the majority of Irish speakers (all Irish speakers that don't speak their dialect) "wrong".
I could explain some of the issues if you gave a specific example of what they are telling you is wrong, but essentially it's like arguing whether "colour" or "color" is the correct way to spell "colour". And just as in English spelling, the vast majority of the exercises in the Irish course on Duolingo will be just fine for speakers of any dialect, it is only certain words or phrases that are markedly different in certain dialects. Pronunciation adds another layer of variation, so even when everyone agrees on the same spelling and grammar, the pronunciation may vary, just as it does regionally in English.