I've found this thread fascinating especially the ponderings on the nonsense words (this being a favorite song of mine)

Interestingly enough the version I heard went something like this (again I might be mishearing a few things...I disremember {O) )

As I was going o'er the far-fled (far-flung) Kerry Mountains
I met with Cap'n Farrell and his money he was countin'
I first produced me pistol and I then produced me rapier
Sayin' ' Stand and deliver for ye were a bold sea man'

This to me especially in the last line seems to justify the robbery a bit, it suggested to me that the robber and the captain served together as mates on a ship or something and somehow or another Captain Farrel owes him the money anyways since he cheated him, stole it, etc. But that might have been just imagination at work. Still it puts an interesting spin on the story eh?