It’s very Scottish-sounding, has very lush harmonies, and, as far as I can remember, it’s mostly a cappella.
I know this is a reach, but I’m hoping somebody out there will know what I’m talking about.
The Incredible String Band (Scottish folk band from the 60s) has a lovely version of Bright Morning Stars. If the one you mean has a female vocal that is really high, then that’s the one.
Hmm, unfortunately not, but thanks. 
The one I’m thinking of has a lot of powerful male voices. I have a vague ghost of a memory that it may be by someone with the first name Alisdair but I may be totally making that up.
Judy Collins did a version of Bright Morning Star with a male backup and the first chorus and last chorus is performed a cappella. It can be found on her RUNNING FOR MY LIFE album. Is this what you were remembering?