Regarding My Lagan Love (which is discussed in another thread that I have not hunted up today), the tune was traditional. Previous words sung to the tune were 'The Belfast Maid'. Hamilton Hearty created the classical arrangement that we hear recorded today. Here is a website with more about Sir Hamilton Hearty. quote: Sir Hamilton Harty has been called "the Irish Toscanini" because of his gifts as a conductor. In addition to his colorful Irish art-songs, he composed several large orchestral works and concertos for violin and piano.
This discussion of the folk process of change reminds me again of the memorable Spancil Hill thread, in which we began with the assumption that it was 'traditional'. Although the tune may have been traditional, lo and behold, the original lyrics and story came to light which contradicted not only the lyrics in the DT, but all the lyrics I had heard recorded and seen in print. The original lyrics and the true story behind it was much more rich and touched me more than the newer mongrels. I find it rewarding to have this ability now to connect through the internet to information that puts meat on the bones of words and tunes I previously knew only from recordings. This ability to tell the story (Joe Heaney - Say the song) adds to the experience of the music. To be able to sit around a group of singers and talk about the song connections adds to the richness of singing the songs. To be able to join a world wide discussion on the internet adds to the experience for me, especially in my rural isolation.