Professor Child died decades before Pádraic Colum wrote his poem 'She Moved Through the Fair', which would be one quite good reason why it wasn't included in Child's collection. The little group of folksongs on which SMTTF was based are only ballads in the sense that they are narrative songs, and that is the sense in which Herbert Hughes used the term, as quoted -without attribution- at the 'Contemplator' website, which should never be quoted as an authority in itself, as the information there is précied, frequently inaccurately or misleadingly, from unspecified sources.

Any suggestion that the melody 'dates back to medieval times' was probably made by a romantic fantasist who also imagined that the pyramids were built by Martians. Of course, I'd be glad to hear of any verifiable evidence that might support either of those assertions; Ossian's Folksongs and Ballads Popular in Ireland books, however, are about as reliable as a dandelion clock when it comes to telling the time.

There are perfectly good definitions of the word 'ballad' as it is used in relation to (1) traditional song and (2) art song. They can be found in any good dictionary, as can the fact that the word is now commonly also used to mean 'a romantic song of any sort' (in all English-speaking countries. I don't see that Irish usage is in any way different from, for example, American). This last is pretty much irrelevant, as it doesn't affect the earlier definitions, all of which are long established and uncontroversial; except when people particularly want to confuse the issue for personal reasons of their own.

We know why the sky is blue (or, rather, why it appears to be); that is a simple matter of scientific fact. What is a mystery to me is why Richard started this discussion. Perhaps he was bored.