@Gordon Jackson: I have always been very careful to spell John's surname correctly, especially for any posters or other publicity for his attendance as guest at our club. Having heard the original songs many times aurally, rather than seen the title written down, I would have assumed that it referred to more than one seaman or fiddler, and therefore the apostrophe would go after the s.
While I don't dispute what you say about how it appears in his songbook, I have his CD from 2013, which is entitled "The Man from Fiddlers' Green"(with apostrophe after the s. Fiddlers' Green is the last track on the CD, and also has an apostrophe after the s! And it also says "notes by John Conolly". So maybe as time has gone on and more seamen and fiddlers have gone to the Green, he has decided that the plural should be recognised?