I just found the following in Brewer's Dictionary of Phrase & Fable :

Fiddler's Green. The land of the leal or "Dixie Land" of sailors; where there is perpetual mirth, a fiddle that never ceases to untiring dancers, plenty of grog, and unlimited tobacco.

Unfortunately my edition of Brewer is undated, but I believe the book was first published in the 1870s, although the work of compiling it must have begun much earlier. So the phrase must have been current at least by the first half of the 19th century.

Andrew