<so I would surmise it's an Irish language contraction>

Wow, isn't serendipity a wonderful thing. I'd never come across 'amn't' before this thread and now I've noticed it at least twice in my current reading: Flann O'Brien's 'At Swim-two-birds' (originally published 1939).

"He sings a lot when he thinks I amn't listening."

"What's the matter, amn't I all right man?"