[RWD version 1 ] How on earth can a walk there and back to a place just over 2k away take over 3 hours?
I had some forebodings about this route having read what Ciarroch said in a colourful comment about doing An Bheann Mhór with Dereenavurrig Hill as a linear walk. He said:"Six hours later we arrived back to the second car as darkness closed in. Impossible to make progress due to rock bench followed by gulley and/or swamp followed by rock bench, and so on and on."So I thought I would do an easy up and down just to the first summit. It's extraordinarily difficult, with endless rocky notches in the ridge each of which is easily big enough to break your leg. The mini valleys frequently have rushes with a few barbed wire fences.
It's easy to reach the mobile mast at top right in the picture above. As you move away from this there are a series of slabs. These are visible from this viewpoint but not when you encounter them. The angle and height of the slabs mean each has to be reconnoitred and dodged.
Just short of the summit there is a particularly big chasm probably 25 or 30m deep.
It's the sort of ridged land that is quite common in the SW, for example on Mount Gabriel (in the Cahas) and Knockanamadane just on the other side of Sneem. I would speculate that in other mountain areas there are rarely the same underlying tilted bedding planes to the rock and where there were they were smoothed by heavier ice than the SW had for much of the Ice Ages.
At least some reward
The 360 degree view of coast, islands, Dunkerrons, Caha Mountains is one of the most spectacular in Ireland. OK, I did spend some time at the top taking photos and measurements but no great proportion of the 3h21 of the walk.
Unfortunately the sun wasn't shining much while I was there, but you can get an idea of the Dunkerron side of the panorama here:
While I was there a white-tailed eagle circled the top for a while. Apart from sheep farmers there are not many signs of people visiting this summit.

All in all a challenge you would want to take seriously.
Of course the challenge is hard because this route goes across the grain of the land.
So if you are considering going up to An Bheann Mhór you might like to experiment with a route that goes with the grain, for example starting from the forest to the north or perhaps from the Glesk (south) side.