The reason for this is, probably, that in Irish the cluster -mn- became disallowed at some point and was obligatorily split by a schwa. As a result speakers of Hiberno English likely didn't have to simplify "amn't" because they pronounced it "ament" from the start.
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.
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.
