The name of the valley probably originates from the Welsh word iau meaning yoke, in reference to the shape of the hill surrounding it.

It is within the Brecon Beacons National Park, in an upland location just below the broad ridge of Hatterrall Hill, which carries the Wales–England border and Offa's Dyke Path.

In 1893, an area in the neighbouring valley of the Grwyne Fawr, known in Welsh as Ffwddog and in English as the Fothock, which had been an exclave of Herefordshire, was transferred into the parish.

Cwmyoy also has a small village hall which is run as a registered charity.

He notes that the whole chancel, not just its axis, is out of line, suggesting it was intentionally built this way and is not the result of the slippage.