Coflein suggests that the "cairn" marked on the OS map is a more modern addition to an earlier barrow:
The cairn is located on the east facing slope, just beneath the top of Disgwylfa. The circular cairn is constructed of small easily-portable stones forming a dense pile and measures 4m diameter and 1m high. The cairn sits within an area of turfed over stones about 8 metres across, the north side of which is preserved as a bank 0.5 metres high and 4 metres long. It seems therefore that the cairn has been constructed in recent times out of an older monument, probably a barrow. The ground cover was heather at the time of survey.
N.A. Vaughan, ArchaeoPhysica Ltd, 30/08/2007