I like the idea of Mary singing a folk song. It sounds much more likely than the more magnificent settings, as it is a protest song after all.
Peter Kearney has been writing songs forever, or at least since 1966 (The Beatitudes). I was astonished to see him large as life at the AOV conference a year ago, when he claimed he must have been a child prodigy. You can hear a nice simple guitar and vocal version of this song here.
Peter has put the text up here. You can also visit his website.
Thanks for writing about this. Our parish has this in our Christmas 2015 hymn list: I did not realise it is in AOV 1/51.
I mean AOV 1/52.