Bobbie -- Search on The Language of Heaven and Michael Lewis - his compilation cd has an instrumental version of Cadair Idris which is Jenny Jones. Perhaps Mr. Lewis, a famous Hollywood writer of music scores, who also has a great interest in Welsh music, could help you.
You might write to some of the Welsh harpists - I'd suggest Rhiain Bebb at Tant Records in Wales - Tant has a web site.
You might also find a tune book with it labeled as Cadair Idris.
I've thought this would be a good song to do, if it had a chorus (more than the first 2 lines above) - I guess I need to write one, unless someone knows of one.
Chris