On the subject of Star of The County Down etc, you'd be surprised how many tunes you can sing Pinball Wizard to.

Evolution is a wonderful thing. I was fascinated watching James May on the telly a couple of years ago, showing how giving a particular computer programme lots of Bach to consider, it "composed" a set of tunes (for piano but you could substitute any key instrument I suppose) that were in the genre, using the cadences typical of Bach.

That principle could more than easily lend itself to jigs reels etc and for that matter, the stanza differences of lyrics that necessitate subtle tune evolution.

A fascinating subject.