The tunes to the two are the same, and although I don't know of any recordings that conflate the words, I have heard people do this live - no doubt for the same reasons as Musicman's: if you like it and it sounds good, do it! I sometimes play "Love Will You Marry Me" (as I think of it, having come to it through De Danann) instrumentally, on the harp. On more occasions than I can remember, people will spontaneously start singing the words, sometimes the "Devil" ones, sometimes the "Love" ones, and often a happy mixture of the two.

It seems to be a similar situation to All The Good Times Are Past And Gone - there are a million different verses & versions which can be added or subtracted as the singer pleases (or can remember). A southern-USA folkie friend of mine used to call them "zipper songs". The good old oral tradition in action, and long may it prosper -