Translating the interjection 'Really?' with 'Go fírinneach?' sounds very odd to me.
An ea? is a perfect translation of Really? It means "Is it so?" and is used where English speakers might say Really?
If you look at Ó' Dónaill's, there are 15-20 words or idioms for the interjection "really." Go fírinneach is better translated at "truthfully."
This is more like 'verily'! Maybe not as old fashioned, but it was not what first came to mind.
Maybe it is my own fault for lingoing the last few months and thinking that watching soaps on TG4 was a substitute for proper grammar work. Alas.
I read here elsewhere that, in Irish, they don't make a special intonation when asking questions. Since this does not seem to have a question particle, how would one tell the difference between "Really?" and "Really."?