"whence did the name come which found its way into the head of this farmer/butcher, who plainly had good links with the broader "folk" movement as they appear to have been watching him for a long time!"

Are you now suggesting that Tom L. learned it from "the broader 'folk' movement"? Any comment, Jim?

There's no mystery: 'St James' was already part of the song when Tom Lenihan learned it. The other examples I've mentioned show pretty clearly that 'St James' was present in a strain of the song as sung in the field going back at least 100 years. The fact that it was found in Appalachia in 1918, and Nova Scotia in 1928, suggests an earlier common origin in the British Isles (unless you can demonstrate a plausible transmission route between the two places). Unfortunately I don't have access to Creighton so can't compare the NS versions with the others.