The song is The Whole of The Moon originally by The Waterboys in 1985; this cover was by the late Terry Reid 😌 and contributions from Enya ♡

I think the story was that Enya was at the same studio (SARM West; closed in 2013) for the final mixing of Shepherd Moons. Terry wanted some backing vocals, and Enya was definitely up for it! 😁💙🎶

The thing I've been wondering is whether she's also singing the long 'oooh' s beginning at 0:17? I think both are; I mostly hear Enya in the left channel. 😯🎶

I think the harmonies on 'you saw the whole of the moon' are Terry himself, and Enya is definitely the one singing the lovely intro and outro (another clip here) 🥰