When I first started out learning the language, I was fortunate to be taught the little bit that I know, by an old person whose first language was Irish. He must have told me dozens of things that I've never seen in any dictionary – the sound of a cock crowing, a way of saying "freckles" that I've never found in any dictionary, etc. I sure wish that I had written down all of that stuff; I've forgotten most of it.

I've even heard an older native speaker (in her 70s) say that that nobody under 50 is fully native.
(She actually said, last year: Na daoine roimh a gcaogad blian, níl an Ghaelainn nádurtha aca, níl an teanga i gceart aca, Béarla a dteanga.)

Obviously this is an extreme opinion, but I know myself that most young (<30, especially <20) native speakers can't really speak Irish at a fraction of their talent in English. They'd really be more like second-generation Italian-Americans or something like that, i.e. they can kind of follow what their grandparents are saying, but not really speak very well past a certain point. Although I've had a girl in Corca Dhuibhne admit to me she didn't really know what her grandmother was saying one time. I think it was because the grandmother used an uncommon word for stream.

My general impression:
Older people know, let's say "stream, river, creek, brook", younger speakers just know "river", you might even be told the other words are Galltacht made up words.

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The dialect I use is Cork Irish.
Ar sgáth a chéile a mhairid na daoine, lag agus láidir, uasal is íseal

If you are looking for books on traditions and folklore there's Scribhinní Béaloidis published by Comhairle Bhéaloideas Éireann - and they're also available from Four Courts Press.

www.comhairlebheal.ie/gaeilge/foilseachain.html

If you were looking for books in Irish published towards the end of the 19th century and early 20th century that are now out of print, there's the Internet Archive.