Re our meeting in Manx Museum - I did find it at http://www.dmm.org.uk/lom/1902_307.htm - Douglas Head Fidlers Green, Douglas Head agent John Cain, 10 The Esplanade, Douglas agent John Cain 2 underground workers no surface workers Lead Ore. Stopped 1902 (I must have been half asleep last night !)

and there is a spot on Douglas head called fiddler's green (Lamplugh gives adit site as Cliff at Fiddler’s Green 700 yards S.W. of Douglas Head) and is described by poetaster J Sutherland as
But we must see the lovely " Fiddler's Green,"
It lies not far from hence-a choicer spot
For pic-nics on a summer's day, I ween,
Could rarely be by any party got.
Here brightly blooms the heather, rich and wild,
While deep below us plays the cresty foam
Among the cliffs in awful grandeur piled,
Where undisturbed the seamews make their home,
And foot of busy man has seldom dared to roam.

doesn't quite sound like a mining site!

re 10 Esplanade - a posh address - a J Cain was there in 1887 though a Capt J Hartwell was there in 1883 - doesn't really sound like your man but it does sound somewhat strange given Lamplugh's comments on the trial adit in 1865/70. Maybe someone can do a lookup in the 1891 census