The subject comes up here from time to time.  Here are a few of those previous discussions which include further information and useful links:

Mouth Music or any Gaelic songs
Mouth tunes
Song Style?
Lilting
mouth music

Be warned, though; some posts repeat the myth that the Wicked English somehow contrived to ban all sorts of musical instruments in Scotland, most particularly the bagpipes.  Never happened, though it's true that local Scottish clergymen (both Catholic and Protestant) sometimes blackmailed their flocks into giving up "the devil's music".  Of course, mouth-music is not peculiar to the "Celtic" nations, being known in almost every traditional culture.

So far as recordings are concerned, I'd particularly mention the School of Scottish Studies'  Scottish Tradition  series, which includes a lot of Gaelic material recorded from traditional singers.

Malcolm