Okay, first step, there are at least 5 songs with the title 'The Foggy Dew' which to me at least suggests the phrase had plenty of currency. I think the 2 Irish ones have known authors and are relatively recent though I'd need to check that.

Of the 3 English songs I would say the likely earliest is very likely a product of the late 18thc pleasure gardens due to its pastoral nature and flowery language. On a Bloomer, Birmingham broadside it is called 'The Brindled Cow' though this version shows some signs of having come from oral tradition. On all other broadsides I have it is called TFD or Foggy Foggy Dew. It is concerning courtship though there is no symbolism obvious. First line in 5 double-stanza fuller versions is 'What shepherd was like me so blest'.

The 4th song I have only versions printed by Pitts prior to 1819. This is a little more risque and is probably contemporary with the printing and a one-off. It starts 'When I was a farmer's son I kept sheep upon the hill' and has 8 3-line stanzas. It is somewhat garbled and typical of a quick hack production of which there are many examples of the period.

Our TFD was widely printed by all the usual suspects around the country, the earliest English copy probably being Pitts but at his post-1819 address. A Scottish version was printed in Kilmarnock 'The Roving Bachelor' round about the turn of the century, but the earliest version I have of the period was a Buga Boo title printed by Wm Goggin of Limerick c1780. Incidentally Haly of Cork printed the longest version of 7sts c1860 and Sanderson of Edinburgh printed a BugaBoo version which I don't have a copy of and his family were printing throughout the 19thc and well into the 20th.

FWIW, and this is my honest opinion, a hack got hold of the Bugaboo version, decided that bugaboo had no current meaning in his area and simply changed it to a popular phrase that also rhymed and was current in other songs at the time.