G'day TonyK,I'm sure you will get a flood of versions from the Irish/Celtic contingent whose song it is, but I note that at least one version is collected here in Australia, far from the Irish mist and ghostly white swans (our swans are black). This came from a wonderful old lady, Sally Sloane who would have learned it from her Irish grandmother Sarah Alexander.
I think this has ended up with a corrupted title "Polly Vaughan", as if it were a surname but the original is an Irish Gaelic word (something like 'bhan' ... and pronounced 'vorn'), possibly an endearment.
I will dig our version out and keyboard in a MIDI/ABC of her tune as well - for those interested in a variant, a long way from home.
Regards and Good Luck,
Bob Bolton