I wonder how the site of the cairn might relate to its landscape. 'Eglwys Faen' means 'Stone Church' and is the name of a big cave in the limestone cliffs immediately below. There is an extensive cave system under here (and shake holes above) - you can read all about it here.
2.5 Metre high cairn constructed with sandstone boulders. 4 metres across at base tapering to a point. To west side of feature large amount of boulders that may have been part of base. Slight 20 cm deep depression noted around perimeter that extends slightly wider than the area covered by the base that suggests that this feature may have been bigger.