They are essentially the same tune. It isn't unusual for completely different songs to share tunes (or vice versa). Unfortunately, Fairport rarely bothered to say where their material came from, and I can't remember just now whether or not that tune was ever associated with Patrick Spens traditionally. Anyhow, it's way past my bedtime. Perhaps someone in a different time-zone will know. Otherwise I'll try to look into it tomorrow. Meanwhile, if you run a search for either song through the search engine on the main Forum page (remember to try alternate spellings too) you'll find out quite a bit about them.