It measures just over two and a half metres long, a little over a metre wide, and barely thirty centimetres high, the kind of dimensions that would make it easy to walk past without a second glance. But that plainness is precisely the point. This is a mass-rock, a makeshift outdoor altar used by Catholic communities during the Penal Laws era, when the open practice of Catholicism was suppressed under legislation that banned priests, closed churches, and made the celebration of Mass a dangerous act.

Mass-rocks are found across Ireland, typically in remote or sheltered spots chosen as much for concealment as for convenience. A cliff angle beside a stream on a north-facing slope, away from cultivated ground and out of easy sightlines, fits the pattern well. Worshippers would gather quietly, a priest would celebrate Mass on the flat stone surface, and a lookout would watch for authorities. The dry-stone construction at Barleyfield, where stones are laid without mortar, suggests it was either built in situ from locally available material or adapted from a natural formation to serve its purpose. The altar's modest height would have allowed a priest to stand behind it and address a congregation gathered on the slope below.