According to Roy Palmer (English Country Songs, 1979), versions of this song first appeared in print in the 1820s and it remained popular in print and in tradition for the rest of the century.  Several traditional versions were found still circulating in Sussex in the 1950s.  Some versions have the "Sun and Moon" lines (often less garbled than in Span's version, the source of which I don't know), others don't: the Rev. Sabine Baring Gould collected one from Roger Hannaford of Lower Widdicombe, Devon, in 1890, and the Hammond Brothers had one from William Poole of Taunton, Somerset, in 1905; this latter is in the DT, here:  The Bird in the Bush

We've tarried here all day, and drunk down the sun,
Let's tarry here and drink down the moon.

Prosaically enough, this would mean, "We've been here all day and have drunk until the sun went down; let's carry on drinking till the moon goes down."  Nothing "Pagan" there, I'm afraid, but it's a nice song.

As Bruce mentioned, there are a few broadside versions at  Bodleian Library Broadsides;  here are two (large images):

Three maids a milking would go  Printed between 1842 and 1855 by Jackson and Son, (late Russell,) of Moor Street, Birmingham.
Three maids a-milking would go  Printed c.1845 by Williamson of Newcastle.

Malcolm