It is a mass-rock, a category of monument that marks one of the more sombre episodes in Irish religious history, and its current surroundings, the Crestwood estate, make it all the more quietly arresting.

Mass-rocks are the improvised altars used by Catholic priests during the Penal Law era, roughly the late seventeenth and early eighteenth centuries, when the public practice of Catholicism was suppressed under legislation introduced by the Protestant ascendancy. Congregations gathered in fields, on hillsides, and in remote places, using flat-topped boulders as makeshift altars. This particular example is a substantial pentagonal granite boulder, measuring approximately 1.9 metres northwest to southeast and 1.7 metres northeast to southwest. Its upper surface is flat and relatively even, sloping gently from around a metre in height at its eastern end down to about 0.9 metres at the west, the kind of natural formation that would have served well enough as a surface on which to lay the items needed for Mass. That it is granite is fitting in a county where the stone is common, though the specifics of how and when this boulder came to be used for worship are not recorded.

What is striking now is less the boulder itself than its context. The surrounding estate has grown up around it, and the rock occupies a narrow green strip as if the developers simply worked around something they could not quite bring themselves to remove. It is the sort of detail that rewards a slow walk through an otherwise ordinary suburb.