The boulder is a mass-rock, one of many across Ireland that served as improvised altars during the Penal Laws era, when Catholic worship was banned and priests conducted Mass outdoors in remote locations to avoid detection. What makes this particular example in Cappercullen Glen unusual is not just the rock itself, but the layered accumulation of stonework that now surrounds it, assembled from the architectural remains of several centuries of religious life.

The shelter was constructed by the Belgian monks of Glenstal Abbey, who gathered fragments of carved and dressed stone from Abbey Owney, a Cistercian monastery at Abington, roughly ten kilometres away. Owney Abbey had its origins in the medieval period, and at least one of the reused pieces reflects that antiquity directly: a 13th-century dressed stone with roll moulding, a decorative technique in which a curved profile runs along an edge or angle, has been repurposed as a leg supporting the rear altar of the shelter. Fragments from three tombs, two from the 17th century and one from the 16th, have been built into the structure's fabric. Two carved stone panels, each around 90 centimetres tall, decorated with incised floral motifs and S-shaped scrolls, are set into the pillars on either side of the mass-rock. These are thought to have come from a wall memorial to Edmund Walsh of Owney Abbey, carved by a craftsman named Patrick Kerin, and possibly linked to the Walsh family mausoleum at the abbey. Two further decorated square stones, built into the adjacent cliff face and used as seats, may also originate from that mausoleum. Other cut stone fragments lie scattered around the site.

The site sits in Cappercullen Glen, where a stream marks the townland boundary between Garranbane and Boarmanshill, with the remains of Cappercullen Castle about 465 metres to the south-west. The woodland setting and the stream make the approach atmospheric in the quieter seasons, when leaf cover is thin enough to let light reach the glen floor. Visitors willing to look closely will find that almost every surface of the shelter repays attention, with carved details that belong, strictly speaking, somewhere else entirely.