What 'Bramicus' read seems to have been based on a misunderstanding by somebody. Colum's poem was first printed as 'She moved thro' the Fair', 'adapted from an old ballad', in Hughes' 1909 anthology (unchanged in later reprints), which also included a song called 'The Next Market Day'. Now, the 'old ballad' is generally taken to be one called (among other titles) 'Next Market Day'; but that isn't the song that Hughes printed, which is more usually known as 'A Maid Went to Comber'. The story is a very different one: see, for example, thread A Maid Went To Comber, where the text is quoted. Note especially the comments from John Moulden and Bruce Olson.

The New Oxford Book of Irish Verse (see earlier post) included a form of 'She Moved Through the Fair' reworked and expanded by Colum as compared to the version in Hughes. It should be noted, as ever, that 'dead love' was no part of Colum's poem; that was introduced later on, via a commercial recording, by the tenor John MacCormack. In the song that formed the basis of Colum's re-working, the reason why the narrator didn't see his sweetheart any more was that she had eloped with another man.

The tuberculosis tradition is actually quite interesting as an example of the folklore that accumulates around songs; there are many such examples, and it can happen very quickly. The reason why Jim's comments were greeted with scepticism and a degree of hostility was that he presented them as bald fact without explanation. He was new here, and probably didn't realise that some songs in particular attract all manner of balderdash from people who don't always know what they are talking about; and that this is one such ('Danny Boy' is another).

His later comments made it clear that it wasn't something he had made up himself but a genuine tradition (even if it originated with Dominic Behan, which wouldn't be a surprise). Whether or not it was what Colum had in mind we don't know; though, given the information available so far, there seems no particular reason to think that it was.