Until today, I only knew this tune though Vaughan Williams' eyes. When I listen to it, I hear the tune O Welt Ich Muss Dich Lassen (in LSB, that would be #880, the tune for "Now Rest Beneath Night's Shadow").
Here is the Vaughan Williams piece.
Here is "Now Rest Beneath Night's Shadow." For those of you with a German Grandmother, tell her Nun Ruhen Alle Waelder and she will probably sing the words while the music plays.
And finally, a straight-up, four-part choral version of Dominus Regit Me.
Dominus regit me: The first words of the 23rd Psalm in Latin. The numbering is different in the Latin Bible, though, so if you look it up, you will find it under Psalm 22.