Thanks for your readiness to help, Bruce and everybody else. I looked through the notes in Frank Purslow's four books, to check on Dan's (Liam's Brother's) reference (and found it) and on the way noticed thanks to "an American colleague, William B Olsen" - you've been being helpful for a long time, Bruce. Frank does indeed frequently allude to Irish origin on little evidence - however, in the case of Fanny Blair, he was dead right. Using all the ballad sheet prints I could find, I conjectured that the incident, which gave rise to the song, happened in the late 17th century, close to the Parish of Tartaraghan, in north Armagh and that the protagonists were Fanny Blair and Dennis Hagan and that local notabilities called Workman, Dawson and probably, Verner (rather than Vernon) were also involved, possibly in some legal fashion.
By a lucky coincidence I was led to newspaper reports of the Armagh summer Assizes of 1785 where Dennis Hagan (formerly a servant to Francis Obré of Clantilew House, in the Parish of Tartaraghan) was condemened to death for the "rape of a very young woman of the county." The newspapers did not name the girl.
Because of the 1922 bombardment of the Irish Record Office, very few older court records survive in Ireland. However, the Assizes Indictments Book for Co Armagh was elsewhere and shows that in the summer of 1785 Dennis Hagan was arraigned before the Grand Jury for the County, three of whose members were called, Verner, Workman and Dawson; his prosecutrix was named as Frances Blair.
Well spotted Frank Purslow.
However, the really interesting thing is that the song is totally unknown, in print or in tradition, in Ireland. Its ballad sheet circulation appears to have been confined to England and northern prints tend to be closer to fact than those printed in London.