A palmer was just somebody who had (or affected to have had) made the pilgrimage to Jerusalem. A sin eater was somebody who took on themself, usually for payment, the sins of the newly dead. Not the same thing at all.

By 'The Palmer Man', I assume that Richard means Martin Carthy's re-write of the Percy MS text of Child 21, set to a tune with which the words had not previously been associated. 'The Well Below the Valley' was recorded by Tom Munnelly in 1969 from John Reilly; the only example ever found in Ireland, and the only tune known to Bronson for Child 21. The two texts printed by Child are the (English) Percy text and a Scottish fragment; neither have tunes.

Presumably Jon Loomes, like everybody else who performs 'Well Below the Valley', is using Reilly's text and tune. Christy Moore can hardly be said to have written new words for it, though he did make some minor alterations. His official website is misleading in that respect, and inexcusably fails even to mention John Reilly.

Sue needs to look at Child's notes for 'The Maid and the Palmer'; these can be found via Google Books nowadays for those who prefer not to buy a print edition. The basic story is an elaboration of the biblical episode of the Woman of Samaria, combined with elements from medieval traditions concerning Mary Magdalen. Such re-workings of biblical stories were common enough. Structural evidence does suggest a familial connection between the English, Scottish and Irish texts; American forms are probably separate treatments of the subject, and have been discussed here separately.