Not necessarily a barrow, but a prehistoric “long mound” according to Coflein:
An elongated isolated mound aligned east – west. The mound is 24 m in length. At the east end the mound is 10 m wide and 1 m high with a slight hollow in the center of the highest point. The west end appears much eroded. What survives is 3 m wide and only 0.2 m in height. Some apparently in situ stone work may be the remains of some form of stone structure at this end of the mound.
Surveyed 2005. Coflein also has some pictures:
coflein.gov.uk/en/site/284527/images/DYFFRYN+MYMBYR%2C+LONG+MOUND/