This lovely pagan hymn was composed by virtuoso guitarist John Renbourn, once a member of Pentangle. The melody is very much in the Protestant hymn-tune tradition, but the harmonies sound older and darker; and the words seem to come from somewhere older still, from times when pre-Christian pagan traditions continued to be mingled in with superimposed Christian beliefs.
Renbourn in fact based his lyric on a prayer titled A Ghealach Ur (The New Moon), which used to be recited on the Isle of Barra in the Outer Hebrides. This was collected in the late nineteenth century by an excise man and folklorist named Alexander Carmichael who travelled widely in the Highlands and Islands. Carmichael made friends with the Gaelic-speaking tinker folk in many places and they shared with him their traditional travellers' prayers, stories, invocations and healing rites. He gathered these together and published them in 1900 as a collection titled Carmina Gadelica.
The words of John Renbourn's song can be found here. Sadly, his original recording of the tune, from the 1988 Ship Of Fools album, is not available on YouTube; but a re-recording from his later Traveller's Prayer CD (1998) can readily be heard.
The Gurt Lush arrangement is fairly austere, mostly in just three parts, but the choir of the Michaeli-Gymnasium school, Munich, sing a rather more elaborate arrangement with the addition of piano and fiddle.
The tune can also be heard in an instrumental arrangement, performed by an unnamed clarinet quartet.