There is an on-going issue with access to this ridge from The Black Valley, however Crossderry and the other mountains may be accessed from the south.
There is parking for a couple of cars at Knocknsallagh Bridge (V84036 78665). Cross the road and then cross a gate, then wade through some long grass for about 20m and you should pick up a faint track. Follow this as it winds its way NNE up the hillside, widening and improving considerably after about 500m, although still with some boggy patches. Pass through a gate around A (V83700 79282) and continue NW along the increasingly damp trail, which begins to peter out around B (V84007 79670). Turn directly north - the terrain from there is a mixture of long grass and heather interspersed with large rocky outcrops, and the slope becomes significantly steeper. Once you reach the ridge the highpoint is about 500m west. The summit area is flattish with pools of mud and the odd rocky outcrop. The highpoint is a slightly elevated heathery area topped by a large boulder. Linkback: mountainviews.ie/summit/488/comment/5247/