Really easy to access this one.
I parked in the large free car park in the village of Ystradfellte and walked north along the road through the village for a couple of hundred yards. After passing a couple of houses on your left you come to the cairn which is right next to the road (also on the left). The cairn is about 1 metre high and 10 metres in diameter. The central section has been dug into and the remaining bank has a ring of trees growing on it.
I visited several cairns in the area today and this was easily the best preserved, despite the damage done by the digging. You could easily have a look by just pulling up along side the cairn; it is so close to the road. There is a fence between the road and the cairn.
Well worth a look when in the area.