I’ve done it from both sides. I don’t really recommend the eastern approach.
The gate was open and there was very busy quarry traffic using the road on the occasion I went up from the east. I still didn’t risk the chance of getting locked in so walked the whole way. It’s a long walk up a very monotonous, uninteresting dirt road to the quarries. In poor visibility, the route up through the quarry is potentially very dangerous. There are many sheer drops and holes which are not identified on the map.
The western approach from Pontsticill is a complete contrast. Lovely views and open countryside.
The only issue (like the other approach) is lack of any visible track for most of the way. There is a track which you can pick up that contours round towards the quarries, but at some time you will need to head off for the summit. I usually follow animal tracks and use my GPS. Without a GPS you would easily get lost in low visibility. It’s a wide open, featureless place.
You can park at the water works, either at the entrance or on the layby down by the river. The bridleway is directly opposite which climbs steeply passing under the Beacon Mountain railway.
Cefn Yr Ystrad is one of my favourite SW summits. Wild, open, remote (after 4 activations I’ve only seen one other person), and stunning scenery (from the west).
My track is on SotaMaps but it differs each time I walk it!
The terrain is like this