The National Monument Record, available via Coflein, gives a very brief description of Cefn Graenog, Cairn I:
A round cairn, 9.45m in diameter and 0.8m high, kerbed by stones up to 0.6m high, overlain by a current field wall on the SE.
The eight figure grid reference given for the cairn is SH45514918 and the National Primary Record Number (NPRN) is 93558.