You may well be right about the false claims of Irish language origin Martin. For one, I don't know of any extant songs written in Irish about Wexford in 1798 (though that may be due to my ignorance more than their non-existence!).

I also found it curious that the alleged 'translation' contained the word 'Sasnach' but no other hint of Gaeilge and indeed that the whole song is in rhyming couplets which would be quite unusual in songs of that time I would have thought - suggesting that it may well have been without a tune.