I saw this cairn silhouetted on the skyline when visiting Gelli Nedd Hillfort. I didn't hike across the moor to get to the cairn as it looked a fair walk and I could see a weather front moving in. It looked (from a distance using binoculars anyway) very much like the nearby Carnau Gwynion cairn which I did visit.
It is hard to give useful directions across open moor land. All I would say is climb one of the rocky outcrops and you should see it against the skyline.