Just a word to the wise: the main route up the Allt Garbh is now on the East bank. We did the route described anti-clockwise, heading more or less directly up to the summit of Aonach Shassuin from the track from Cougie, which is now enlarged to service a run of the river hydro scheme. The descent from Na Cnapan at the end of the day is a nightmarish slog through chest deep vegetation and bog. In a couple more summers it will be impassable. If you're not too bothered about the rather nice little rocky top of Na Cnapan, the easiest way would be to follow the hydro road to near the hut and head directly up to Beinn Iochdarrach, then follow the route as described.