There is no evidence beyond wishful thinking that this was ever a "pagan" song; the above "variants" are modern inventions, I'm afraid.  The song was first published in a collection in "Joshua Sylvester"'s Christmas Carols (1861); he said that he had got the text from a broadside of c. 1710, which seems to be as far back as the song can be traced.  You can see a couple of early 19th century Birmingham broadside copies at  Bodleian Library Broadside Ballads;  the texts are pretty much the same as those found in oral tradition, all of which, so far as I know, contain that merry organ.  Well, one of the broadsides has a merry groan instead, but the typo doesn't seem to have made it into tradition!

The tune in Sorcha's link is a French one, not traditionally associated with the song.  The midis with the DT file are from tradition, though it isn't made clear which is which, or which belongs with the text given.