This isn't much help, but I haven't heard anyone say Southwind may have been composed by Carolan before. It's not in the book I have. I think many things may sound like his works because he grew up with the music and was inspired by it - it certainly does have a Carolan-ish feel to it.

Could this be one of the rare cases where the words and tune were composed at the same time? There are English lyrics and some information at the Slow Airs page. (It also discusses the song.)

Thanks for asking about this tune - it's one of the loveliest ones I know, and I'd love to learn more about it.