What exactly does "do" mean here?
It represents the English "to". Inis do means "Tell to".
Example: Inis dhom faoi do mháthair. -> "Tell me about your mother."
Nach bhfuilim in ann 'nior dhuirt' a usaid anseo, ce nach bhfuil se chomh cruinn le 'inis', b'fheidir?
That's not quite the same as tell. It's you didn't say
Interesting... The hover-text gives a conjugation of the verb "Abair" instead of "Inis" here. I'm glad I went with "Inis" anyway because the sentence uses "tell" and not "say."