It measures just over twelve metres long and barely two metres wide, which gives it less the feel of a building than a channel cut into the landscape. At its eastern end, a single flat slab laid across several supporting stones forms a simple altar. The place is known locally as Clais an Aifreann, meaning, roughly, the hollow of the Mass.

Mass-rocks were the improvised places of Catholic worship that came into use during the Penal Law era, broadly spanning the late seventeenth and eighteenth centuries, when Catholic clergy were forbidden to practise openly and the celebration of Mass was a criminal act. Congregations met in remote or sheltered spots, often with a lookout posted on higher ground, using whatever flat stone was available as an altar. The site at Cappaboy Beg is a particularly well-defined example. The rectangular enclosure, with its small opening at the north-east corner wide enough for a single person to pass through, and the secluded natural hollow that contains it, suggest a place chosen and then quietly shaped for repeated use rather than a single desperate gathering. The hollow itself runs roughly forty-two metres on its longer axis, providing natural concealment from the surrounding hillside.

The altar stone, one flat slab resting on several others, is the kind of arrangement found at many such sites across Munster, improvised in appearance but clearly deliberate in construction. What distinguishes Cappaboy Beg is the degree to which the built enclosure has survived, its walls still reaching around eighty centimetres in height, preserving something of the spatial logic of the original gathering place.