Having now reread the whole of Roly Brown's article mentioned above, it appears we do have the facts of the case or at least contemporary descriptions. These have been researched by John Moulden and the trial and execution were reported in the Belfast Mercury or Freeman's Chronicle, Vol III, No 19, Tuesday October 4th 1785, p3.

The ballad, like many other N Irish ballads, appears to have entered England through the port of Liverpool. The earliest and fullest version appears to have been printed by Armstrong of Liverpool in the 1820s, and from there spread around northern England, a truncated version ending up in the south and printed by the likes of Catnach and Pitts in London.

I have a copy of John's thesis, 'The Printed Ballad in Ireland' and as soon as I get time I'll check out the info he gives.