After singing my own mix of the Nic Jones and Christy Moore versions at a festival one year, Jerry Epstein asked where the tune was from; I said I learned it from Nic Jones, and he said it was the tune used by the American ballad-monger (mangler?) John Jacob Niles, who had the unfortunate habit of copywriting all his material, traditional or otherwise. I don't know if Niles' tune was trad. or not; but it would have probably been an American variant. I haven't bothered to go look up Niles' version to see if Jerry was correct; I love the tune, and whether it's English, Irish or American doesn't matter to me that much. Christy Moore's set of words are fairly close to one of the variants given in Child; he leaves out my favorite ending, where Lord Arnold/Darnell tells his men to

"Bury my lady at the top
For she came from better kin."