At least in Kerry, there's a tendency for /mna:/ (Ó Sé Gaeilge Chorca Dhuibhne, 33) and he explicitly states (32) that both /mn/ and /m'n'/ are legal consonant

At least in Kerry, there's a tendency for /mna:/ (Ó Sé Gaeilge Chorca Dhuibhne, 33) and he explicitly states (32) that both /mn/ and /m'n'/ are legal consonant clusters at the beginning of words, no epenthesis (he gives 'don mbeannacht' as an example for the other, where the vowel is dropped). I'd suspect the rest of Munster to be the same.