Coflein says....
A disturbed round cairn, 13.1m in diameter and i.52m high, having a lesser cairn set upon its summit.
Postman says....
I didn’t go over because time had run out, far too much sitting around gawping. It is over half a mile from the bigger pair to the east, as such I’ve added it as a separate site. None of the cairns are at the summit of Gyrn Ddu but rather on lower subsidiary peaks.