There are lots of folk and old popular tunes in 18th century plays and operas, but not, to the best of my knowledge, any folks songs. Sam Arnold, Jr. rewrote "Our Goodman" for 'Auld Robin Grey' and set it to an old popular tune. Samuel Arnold included many folk tunes and usually gave the original titles of them. Wm. Shield didn't. Kane O'Hara didn't either for the folk tunes in Midas or the Golden Pippin. [For these and others see Roger Fiske, 'English Theatre Music in the 18th Century', 2nd. ed., 1986]