I feel you're more likely to hear i ndáiríre in this case rather than go fírinneach, however.
So, you'd use "i ndairire" in the case of, not really believing what another one just said? And when would you use "go firinneach"?
Go fírinneach seems to be more of an adverb meaning “honestly” or “truthfully” than an interjection analogous to “Really?” (if this exercise was intended to express an interjection).
Truthfully -Not accepted as a translation. 14/03/17. Truly was suggested.
It comes from the word truth, so truthfully should be accepted in addition to really.