Looking just a tad further, commentary on the Piae Cantiones says that over half the songs are believed to be of Finnish (or Swedish) origin, having been encountered in no other works, and that the collection was unknown in England until 1852. The Vikings may have scavenged some materials incorporated into the Cantiones, but the work itself was Finnish.

Thread creep:
The collection contains 74 songs; there are at least 6 folios that I've seen referenced. The facsimile collection linked above only reproduces 31 of the songs (2 folios?, the first being the Christmas songs). "Tempus..." is not among them, belonging to the third folio.

John Mason Neale freely translated, or set to new lyrics, twelve of the Christmas songs, published in his Carols for Christmas-tide (1853). These included "Good King Wenceslas", "Christ was born on Christmas day" and "Good Christian men, rejoice". Neale followed this with twelve more adaptations comprising Carols for Easter-tide.