The - mn - cluster is legal in Irish -- the plural of bean (woman) is mná even. It occurs a few other places too, though some dialects have gone away (though any Cn has become Cr unless it's sn, so not just mn) from it.
AFAIK, mná is never actually pronounced in this way, it’s either [mena] with an epenthetic schwa or [mra].
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.
Cool, thanks! But word-final -mnt is a slightly different thing anyway.