According to the article link that David T posted it appears that James Braid created or built up the dune in 1923 prior to the Open.
The first iteration of the hole was the work of Willie Fernie, then in preparation for Troon’s first Open Championship, the course was refined by James Braid. The 8th got shorter, bunkers were added, as was a large mound beside the green, in short it became a little devil.
If that is the case then the setting of the hole is at least partly or mostly man-made.
Below is a picture of the hole from the 1923 Open from the Mackenzie and Ebert Royal Troon Brochure which would have been from shortly after Braid made his changes. It looks very little like the conical dune of the painting. Certainly, no dune would naturally exist or last in such a conical form. From the picture from 1923 it doesn't appear that the dune as built was conical.
Bryan,
I've had an interesting exchange with Colin Cotter off line regarding this hole. Colin's family have been members at Troon forever and he has caddied there for years. More to the point, there is probably no one who has done more research into the evolution of both courses.
Colin agrees with me that the winter green post dates the postage stamp green but thinks that it possibly dates back to before WWII. He agrees with me that the Braid changes were "restricted" to putting in the left hand bunker. IIRC articles at the time suggest the reason was to stop players deliberately aiming for the left side of the green knowing that if they do hit it too far left the ball will run off the bank and on to the green. So a quite significant change on how the hole played even if there wasn't huge amounts of earth shifted. And the bank/dune was/is definitely natural.
In terms of yardage, I haven't checked to see how the yardage changed over time but I wonder if any changes could be accounted for simply by the way the hole was measured ? At one period of time (pre-WWI and just after) the distance was sometimes measured from the back of the tee to the back of the green. Once you start measuring to the middle you lose probably 10/15 yards.
One further comment, in that old photo the left hand side bunker(s) are above the green whereas now the bunker is most definitely below the green surface.
Niall