A plaque fixed to the rock records it as the traditional site of a mass cave, a sheltered hollow or recess in the landscape where priests celebrated Mass out of sight of the authorities during a period when doing so was a criminal act.

The Penal Laws, which reached their most restrictive application in Ireland during the late seventeenth and early eighteenth centuries, banned Catholic worship, the education of priests, and the public exercise of the faith. In response, communities gathered at outdoor locations, including flat rocks in open fields, cave mouths, and sheltered riverbanks, wherever a degree of concealment and a lookout could be arranged. The site at Walterstown belongs to this tradition. The plaque erected here in September 1863 does not claim to be the rock itself but rather commemorates the memory of such gatherings, describing it as the place where "the Holy Sacrifice was offered during the Penal period." The date of the plaque is itself telling: the mid-nineteenth century saw a wave of Catholic commemorative activity across Ireland, as communities marked and memorialised the landscape of suppression now at a generation or two's remove.

The location, next to a cliff face overlooking the river, would have offered practical advantages to those gathering in secret: a natural windbreak, a clear sightline across water, and the kind of marginal ground that drew little official attention.