Its walls, double-faced and built to last, stand no higher than sixty centimetres now, but their careful construction tells a different story. Locally, this place has been remembered as a Mass-station, and that memory carries considerable weight.

Mass-stations were informal gathering points used by Catholic communities during the Penal era, when the public practice of Catholicism was suppressed under a series of laws enacted from the late seventeenth century onward. Congregations would meet in remote or inconspicuous spots, often outdoors, to hear Mass said over a flat stone or simple makeshift altar. The structure at Croaghan fits comfortably within that tradition. Its modest dimensions, roughly six metres east to west and five metres north to south, with a doorway just over a metre wide set into the western wall, suggest a gathering place rather than a formal ecclesiastical building. The site is identified by the historian Ó Gallachair, writing in 1957, as probably the Tassan mass-rock, linking it to a specific named place in the local religious geography of Monaghan. The rock outcrop on which it sits would have made it both visible to those who knew where to look and unremarkable to those who did not.