I've been doing some work on "Green Grow the Rushes O" and the Dilly Song. Mostly, this is simply bringing together stuff on the Web (some of it from Mudcat!) but I've a few versions not found in Mudcat. The whole thing runs to 40 pages (currently) and anyone who'd like an electronic copy, please backchannel me. A short summary follows.
It looks as if "Green Grow the Rushes O" and The Dilly Song are fairly distinct poems (while they both refer to the lily-white boys, they diverge markedly after the first few lines) rather than simply different versions of the same poem. (I've managed to identify six versions of "Green Grow the Rushes O", one from Scotland, and include three in the material.)
The "best" text (certainly the one with the strongest riddle elements) comes from Dorset, and seems to be written down for the first time in the late nineteenth century, but it +may+ predate that in the oral tradition. Most people seem to think so, but I'm not absolutely convinced - it could be a Victorian pastiche, though the variety of versions goes against that. Except "the lily-white boys" phrase only occurs in two versions of the text. My feeling is that The Dilly Song is a late rewriting, taking off from the beginning of "Green Grow the Rushes O".
If I were to attempt to characterise "Green Grow the Rushes O", I'd describe it as a counting song, with elements which are a mixture of the biblical and the cosmological, with a strong riddle element. The Dilly Song, it seems to me, is later, and substitutes for most of this a mush of pseudo-medieval (if rather resonant!) elements such as the Gilly Song, the Dilly Hour, and the Gilly Bird. All a bit twee, if you ask me. But maybe exactly what the Dilly/Gilly stuff is constitutes a separate set of questions.
I've included as many texts and more or less closely related poems as I can find, and some of the interpretations that appear on the Internet.
I think I'm fairly happy with the interpretations of all the lines except the lily-white boys and the six proud walkers. Phoey!!
About the only original contribution on my part is towards the end, on W.H.Auden's relation to the song, and on a possible link with a 17thC poem, "Tom o' Bedlam's Song". I haven't really argued this through properly, as I finished work on the current piece at 3 am this morning (!!!).