It can happen that a language starts to be transmitted to children. Think of creoles. So new living languages can come into being.
What about him? Once upon a time there was a living language of the Jews. Then they adopted Aramaic instead, and Old Hebrew became dead, even though it continued as a written language. Some two thousand years later a version of the written language started to be passed on to kids and became the living Modern Hebrew. BTW, I don't know how different it is from living Old Hebrew, but I think the discontinuity should be evident.
In idem flumen bis descendimus et non descendimus. Imagine that at our time some community adopts Latin as the everyday language and the children become native speakers. Personally, I find it easy to believe that in, say, two generations kids will be having a hard time reading Cicero and their Latin would terrify any classical philologist. So if by miracle all the anglophone Ireland switched to Irish, I still believe the Irish that we know now would cease to exist. In this unlikely situation a new form of Irish is likely to emerge.
But what about Hebrew and Irish? Modern Hebrew came into life not through foreign studies, not through textbooks like "Hebrew in 30 days" or "L'hebréu sans peine", but through devoted ... (I don't know, devoted
who. Probably, not
fans? Anyway, Ben Yehuda was
devoted above average by all accounts) who were eager to
replace the vernacular in their families by Hebrew. So I don't see how eventual foreign students would help Irish. Granted, if my family moved to a Gaeltacht, if my kids went to Irish kindergartens and schools, if I encouraged their communication with other kids in Irish, then I assume I would indeed contribute to the reviving of the Irish tongue. Even if my children didn't qualify as native speakers, my grand-children probably would. But somehow, I don't feel the motivation to do so. After all, for the Jews Hebrew was a national symbol, the language of their religion and culture. Whereas Irish doesn't motivate much even the Irish, and I'm not one of them.