Q is correct - probably unlikely that it is older than 18th Cent. Though, of course the traditions that it connects with can be partially Christian and Pagan. Chrisitanity adapted to the local folk traditions wherever it was introduced, made some of them its own, and attempted to forget other ones. Of course, not all were forgotten by the converted masses and some got passed down through story, lore, poetry and music.

Also, in sokme pagan traditions, there are two kings which rule the two seasons each year. The Holly King and the Oak King. There are parts of the British Isles where the Oak King is also known as the Ivy King. I don't know if this has anything to do with the song, but you never know.