Route over the two Coomcathcuns, An Bhinn Lair and Cnoc Breasail.
Parking is in the farm yard used by Simon3 in track 2457 and Colin Murphy as mentioned on the Coomcathcun South Top hill page. The descent is on a track which runs right to the col between Knocknagantee and Cnoc Breasail. This is a serious construction with little obvious purpose which rises to c.600m and is wide enough in places for two vehicles to pass. While a normal car would be pushing it, a 4x4 could probably make it all the way. Note however, there are two gates at the bottom. I had used this track before on a descent from Knocknagantee but had not realised how high it goes; it is potentially an easy way onto, as well as off, these hills too.
Utterly brutal terrain means this is a tough walk and I found it a little frustrating in that there is little break from constant changes of direction and elevation; there is literally nowhere to stride out and cover real ground; every inch has to be fought for.
It is worth calling attention to one part of the route; the descent from An Bhinn Lair towards the lough which lies in the col between it and Cnoc Breasail. This is a potentially lethal descent and huge care has to be taken. I have never been crag fast, but if it is to ever happen, here would be as likely a place as any. The descent, at least as I made it, involved lowering yourself onto ledge after ledge, searching each time for the next ledge and always without clear visibility of the entire descent. I am sure I made some bad choices and maybe if I had taken a little time and studied the entire descent more closely I might have found a better way. A cursory glance at the OS Map, will show you very confused contours around these parts, so a bit of probing around on the ground before committing to any particular descent would be in order. I did make it down but I definitely won't recommend that anyone do it the way I did!
[I subsequently took a good look at Captain Vertigo's track 2495 which covers the same descent - unfortunately while I was aware of this track, I hadn't gone as far as to download and take it with me. While the Captain seems to have had an easier descent, though not any kind of 'doddle' as his cautions make clear, overlaying the two tracks reveals only relatively minor divergences, meaning careful route finding at a micro level is critical.]