Visited 23rd September 2003: This cairn is a bit of a mess, but represents the last survivor of a number that once stood in the area (according to the NMR). There's a gateway opposite the cairn, but the cairn material isn't easy to see without taking a closer look.
Edward Lhuyd gives us to understand that the parish church of Cellan, in Cardiganshire, which he writes "Keth-Lhan," is dedicated to [St Callwen], and that there is a spring there called "Ffynnon Calhwen." All Saints is the dedication now usually given to the church. On one of the mountains in the parish is a cistvaen called Bedd y Forwyn, the Virgin's Grave.
Maybe this is the right place. Coflein doesn't mention the name. But The Cambrian Traveller's Guide says "Upon the mountain to the N. of the river Frwd, are two beddau or graves; and on an eminence to the S. are two more, one of which is called Bedd-y-forwyn, or the Virgin's Grave." So it's in the right sort of place.
Edward Llwyd was writing in the 17th century.