Not too many tracks posted in this area. This starts from the road south of Ballymurphy and follows a tarmacadamed lane eastwards past a single farmhouse. When the tarmac ends you find yourself on a green road which zig-zags up the hillside, past some ruins to the broad col between Blackstairs and Carrigalachan.
I parked on a grass verge along the main road as there was no other indication of parking spaces on the East-West map of this area. This potentially added to the distance.
The first section of the "green road" appears little use and is being overtaken by bracken.
When at the col, rather than turn northeast immediately, walk eastwards towards the gate at the edge of the forestry before turning north on a noticeable track.
A better (and possibly more frequently used) startpoint could be S 79220 45123, but parking would appear to be scarcer there if the map is anything to go by.
To make a loop of it you'd head north to Scullogue Gap and return via country lanes. You'll find a description for same in J Lynam's Best Irish Walks.