Having waded through the posts and being still not much wiser, I have to agree that the bare bones of the story is what makes it haunting and captures our imagination, distinguishing traditional folk songs from more anecdotal ballads.

I don't know much about medieval Irish culture but we only need to look at cultures around the world today to imagine that perhaps this young woman was not in charge of her own destiny and had naiively misjudged her family's reaction. She does not appear to have actually broached the subject of marrying the impoverished young man so perhaps she went home and dropped the bombshell. It also seems rather strange for a young woman to be going home from market alone in the small hours of the morning carrying her goods - was she attacked? Maybe she was already pregnant, maybe she was hastily married off to an older man (the thistle in the longer version?) Maybe she was unhappy at being unable to marry her young love and killed herself or maybe she died in childbirth. There are no end of possibilties but it does feel more like a song of thwarted love of the Romeo and Juliet genre than one of betrayal. It invites the listner to fill in the gaps.