Dona Nobis Pacem
Canterbury Fair sang the round Dona Nobis Pacem on their eponymous 1977 album Canterbury Fair. They noted:
This appears in Reprints from the People's Song Bulletin, 1946-1949 (Oak Publications 1961), and is a rather beautiful round. The words mean ‘give us peace’, which is a sentiment we all fell very strongly, and for this reason we would like to conclude our record with the song. Unfortunately, the origin of this sing is unknown to us, though we assume it to be mediaeval.
See also the Mudcat Café thread Origins: Dona Nobis Pacem (round).