It measures roughly 1.9 metres east to west and just over a metre north to south, rising to about 75 centimetres from the ground. A smaller boulder sits immediately to the east, half-buried in overgrowth. To most passers-by it would read as unremarkable field furniture. Local knowledge, however, identifies it as a mass-rock, which places it in a very particular chapter of Irish history.

Mass-rocks are outdoor altars, typically natural flat stones, used by Catholic communities during the Penal Law era, broadly the late seventeenth and eighteenth centuries, when the public practice of Catholicism was suppressed and priests faced severe legal penalties. Congregations would gather at remote or inconspicuous spots, often on hillsides or in sheltered hollows, to hear Mass said in secret, with lookouts posted against the approach of authorities. The south-facing slope at Glanarough would have offered some shelter and reasonable sightlines, the kind of practical considerations that mattered when a religious gathering carried real risk. Many such sites survive across Ireland, though they are often known only through oral tradition passed down in the local community, which is precisely how this one has been identified.