South West of Ballynahinch, Dromara. Park outside the Forest on the SW edge, or on the West side of Slieve Croob.
Has not got anything specifically designed for mountainbiking, but there are singletrack ponytrekking tracks at either end of the forest (which can be seen on the map at 1:25,000).
One connects two fireroads, and the other (whilst unrideable to the top) is a nice singletrack back down for about 500 meters.
Slieve Croob itself is best accessed by the tarmac path on the West side. The Drumkeeragh side is soft heathery peat, and not rideable up, but is a fine technical challenge if you meet the path just below the saddle between Croob and Slievenisky.