Ah well, there are at least 2 categories of folk song
1) Our heritage, what has been passed down through the oral tradition and preserved on recordings of ancient singers - the academic approach.
2) Songs that people love to sing, regardless of how old (ie new!) they are, how they got them and where they came from. (eg, the Sidmouth Anchor Middle Bar approach).
Plus the introspective singer-songwriter-guitarist approach that is ephemeral because no-one apart from the author sings the song ever again.
As I've mentioned on another thread, Ossian published 4 books of songs popular in Ireland. They all get sung in Ireland, but they didn't necessarily originate there.