I think it is possibly a common sort of throw away ending from another song that got detached from somewhere else. Remember oral traditions do tangle up songs and tunes and both or either,songs and words are often much older than the point at which they were collected or given lyrics- look at the better known tune to 'Ae fond kiss' and ' Rory Dahl's Port or ' Dannny Boy' and the ascribed 'Derry Air', again to one Rory Dahl.
However the wish for the army to return may , unintentionally or not, reflect the impossible wish that Time can be reversed and mistakes, such as the one rued by the 'false young man'here, put right.
I am trying to build a story around this song, and am largely influenced by the sad tune evoked by Andy Irvine on his hurdy gurdy, which is similar to the haunting noise made by eilean pipes and is of course, one might say, 'very Gaelic' It is very sad and regretful in this case. As to religious differences, well it is just a speculative piece of licence om my part. The original song may not even come from nineteenth century Ireland, where the 'lassie that has the land' might well have been an Anglo-Irish girl from 'the big house'- but there is no clear evidence of that, or of the period in which it is set. Like all good art, its message is universal
The 'misfortune' might well have been some sort of circumstance that forced him into revoking his vows and marrying the wrong woman or it might well have simply been a wrong but uncompelled judgement by a selfish soul. We have to use our own imagination.
Anyway, as sung by Andy Irvine and Planxty all those years ago it is a hauntingly sad and beautiful song telling 'a sad old story' perhaps reminiscent of Hardy. Let us just enjoy it for what it is.