In his Notes on Irish Words and usages, PUL said Magh Chromtha is never lenited (never Mhagh Chromtha), as an exception. It is very odd, as I have found it lenited in his works - but Notes on Irish Words is a compilation of his comments, explaining passages of his works, in the Cork Weekly Examiner 1910-15, so he must have failed to lenite it in one of his works, and came up with this explanation. Is there a possibility that Magh Chromtha should not be lenited?

Well, if he says so it probably is true but bear in mind that this is just one individual's opinion. However good PUL is he is just one speaker of the dialect. You would need to find the opinions of others.