Irish is a Celtic language and the native language of Ireland. However, the vast majority of Irish people now speak English as their native language, and most don’t speak much Irish at all. It is still spoken as a language of daily life in the Gaeltacht areas.
It declined over about two generations in the 19th century during the time of the famine, and these days there is a schism between “Gaeltacht Irish” and “urban Irish” – the latter group missing slender and broad consonants like half the time meaning important inflections etc. are being lost from their speech, and they also exhibit less complex syntax including way fewer subordinate clauses, although their vocab is roughly on par with Gaeltacht Irish people’s. You could say this demonstrates a potential pitfall of language revival, but I also don’t think it’s like this pitfall outweighs the merits of bringing a language “back from the brink” in the first place. It’s just that, probably, great care needs to be taken to ensure that Gaeltacht speakers are not “drowned out” by the increasing numbers of newly Irish-speaking urbanites.