This cairn is listed by the Gwynedd Archaeological Trust at the grid reference SH66391355 as ‘cairn’ of a period ‘unknown’. It looks like a ring cairn, but according to Coflein it’s usually enterpretted as a damaged round cairn:
A round cairn, 22m in diameter & 1.0m high, defined by a stoy bank, thought to be a disturbed round cairn, rather than a ring cairn. Has been suggested to have been a cockpit.
The cairn isn’t marked on the Ordnance Survey Landranger map, but it is marked on the Explorer (number 23).