It is easy to overlook entirely. The uneven ground is scattered with fragments of granite masonry, an iron railing along the west side bears a cross emblem, and a single doorway, just under a metre and a half wide, opens to the east. What you are looking at is, in all likelihood, a Mass house: one of the small, clandestine buildings used by Catholic communities during the Penal Law era, when public Catholic worship was suppressed and priests operated at considerable personal risk.

Documentary evidence suggests the structure dates to 1697, according to research communicated by Tony Duffy, placing its origins in the decades immediately following the Williamite wars, when enforcement of the Penal Laws was at its most severe. The site itself occupies a D-shaped plot enclosed within a bend of the Owendore River, a natural boundary that would have offered some degree of shelter and concealment. By the time the historian D'Alton came to describe the later chapel that replaced it, he characterised that subsequent building as a cruciform edifice with galleries, disproportionately low, suggesting the congregation had grown considerably from whatever huddled in this small square room. That larger chapel served the community until 1878, when the Church of the Annunciation was built across the road. Adding to the accumulated material of the site, two stone cross-shaped finials and a wrought iron cross finial were found on the grounds of the nearby presbytery, close to the present grotto.

The remains sit within the grounds of St Mary's parochial house, so access is not simply a matter of walking in off the street. The curvilinear raised area, roughly 18 by 20 metres, is partially defined by a low masonry wall and the river bank, with a shallow linear hollow marking its southern edge. Anyone with an interest in early modern Catholic history, or in the archaeology of religious persecution more broadly, will find the understated quality of the place rather striking. There are no interpretive panels, no formal presentation. The cross finials discovered nearby, now held close to the grotto, are perhaps the most tangible remnant of whatever ritual life once animated this small, unroofed enclosure beside the river.