Hi,
According to Lesley Nelson (a.k.a. The Contemplator), the rebel version of The Foggy Dew was written by Canon Charles Nelson, a parish priest in Kilcoo. See, that page of the web site at:
www.contemplator.com/folk4/foggydew.html
I still have to learn to do that blue clicky thing.
Anyway, according to the information at that page, Canon O'Neill was moved to write the song after attending the first meeting of the Irish Parliament in 1919. The page also says that it was recorded by John McCormack in 1913, but that is incorrect. According to the John McCormack web site, he recorded it in 1923.
So it appears that the Foggy Dew (rebel version) would have been written sometime between 1919 and 1923.
Hope that helps,

Robby