During yesterday's gospel, this song leaped into my head. I became aware of this old carol just a few years ago, but it's now one of my favorite Advent hymns.
Updated to answer a private comment: Yes, I know the Immaculate Conception refers to Mary's conception without original sin. This song sounds like it's more about Jesus' conception/incarnation, but it follows the gospel reading for December 8th. "The angel Gabriel was sent from God . . . " Hailing Mary as "full of grace" and (in the song) "most highly favored lady" reminds me of her sinlessness. Anyway, happy listening!