In An Gort Broc, a collection of folklore from a single speaker in Beara, it’s clear that the language had already passed out of common usage in the area and the speaker was one of the few left who could speak it well. Nevertheless loss or sporadic use of lenition where one would expect it to occur shows some weakening of the speaker’s own Irish, isolated as he probably was from other speakers.
There is a similar sort of effect to be seen in “Seanachas ó Chairbre”. Seán Ó hAo, the speaker used in the work, often misses lenition and eclipses and frequently uses the nominative singular form of adjectives regardless of case and number (although not inflecting the noun for case is a common thing in Munster I think). However there is an indication that he knew the “correct” forms and also some indications that what seem like errors are features of the Cairbre dialect going back to when the area was completely Irish speaking.
Really good book by the way, the speakers vocabulary is enormous and there is a rake of words not be found even in Dineen.