"It is me really."
Translation:Is mé go fírinneach.
In the realm of “should”, it should be “I” rather than “me”, since a nominative complement is what’s needed in the English sentence. (Colloquially, I know that “me” is by far the more popular choice.)
In the Irish translation, the lack of a complement is unusual with is ; the only circumstance that comes to mind where a complement wouldn’t be needed is in an answer to an identification question, e.g.
Peadar: An tusa an tUachtarán, le fírinne?
Pól: Is mise go fírinneach.
but that would be more of an “I am” than an “It is me”.
Regarding your comment about how it should be "I" in the English sentence: With respect, I think this is wrong. As far as I kmow, only according to pedagogical grammar schemes contrived by people who thought English should have Latin grammar is it correct to say "It is I". Imagine I point at a photo of you and say, "Who's that?" Would you answer, "It's I." English grammar just doesn't work like that, and there's no reason to think it should. Saying "It's me" isn't just "colloquial", it's 100% correct native-speaker English.