'Gurney': since all 'versions' of 'She Moved Through the Fair' are based on Colum's, it might be best to consider his the original rather than merely as another 'version'; though as we know he based his poem on elements of existing material, these told rather a different story. It's certainly true that a lot of silliness has resulted from McCormack's (possibly accidental) alteration to the song; though it certainly gave it a new resonance for a lot of people, its author hadn't intended, so far as we can tell, any such thing. I wonder if anyone ever asked him what he thought about the whole business?

I don't need to read Herriot to know about TB; my father contracted it in the early '70s. Because innoculation and other precautions had largely eradicated it in Britain at that time, diagnosis was late and he sustained quite a bit of permanent lung damage before he got the right treatment. The medical establishment hadn't fully appreciated then that increased immigration from countries lacking proper innoculation programmes would inevitably bring with it an increase in diseases of that kind.

As one theory among others it's fine; but without evidence it can only be speculation. Consumption has for centuries served as a romantic plot device, though the reality is very far from romantic. Since there is an Irish connection here, it's worth mentioning that TB was a recurring motif in Van Morrison's early work; sometimes the references were direct, sometimes more oblique. That isn't guesswork, though.