The incident on which both versions of The Croppy Boy is based is recounted in Zimmerman's 'Songs of Irish Rebellion' p.229. He quotes from an anecdote in William John Fitzpatrick's book - "The Sham Squire;" and the Informers of 1798, with a view of their contemporaries. To which are added, in the form of an appendix, Jottings about Ireland Seventy Years Ago'.

"About the same time, and in the same county, [Wexford] the yeomanry, after having sacked the chapel and hunted the priest, deputed one of their corps to enter the confessional and personate the good pastor. In the course of the day some young men on their way to the battle of Oulart, dropped in for absolution. One, who disclosed his intention, and craved the personated priest's blessing, was retorted upon with a curse, while the yeoman, losing patience, flung off the soutane, revealing beneath his scarlet uniform. The youth was shot upon the spot, and his grave is still shown at Passage."

There are several online editions of "The Sham Squire": the passage quoted above may be read on page 196 of the 1866 Boston edition which is reproduced online in The Internet Archive at The Sham Squire.

Zimmerman also commented in a footnote (p.39) about the term "croppies":- 'In the 1790's those who admired the Jacobin ideas began to crop their hair short on the back of their head, in what was said to be the new French fashion; in 1798 this was considered as an evidence of disaffection.'