Incised into the rock in the manner of late eighteenth or early nineteenth-century gravestone lettering are the initials "BM", standing for Blessed Mary, and that detail alone locates this modest stone within one of the more quietly dramatic episodes in Irish religious history.

Mass-rocks are the physical remnants of Catholic worship conducted outdoors during the Penal Law era, when legislation barred Catholics from practising their faith in formal church buildings. Priests celebrated Mass at improvised altars in remote or secluded spots, often with lookouts posted against the risk of discovery. The Derrydarragh example is a roughly shaped limestone boulder, approximately 1.15 metres along its north-east to south-west axis and 0.9 metres across, rising to a maximum height of 0.65 metres, dimensions that suggest something functional rather than ceremonial in origin. It sits around 33 metres to the south-south-east of a holy well, a proximity that is unlikely to be coincidental. Holy wells had long been focal points for popular devotion in Ireland, and the clustering of a mass-rock nearby suggests that this small corner of County Longford served as an informal sacred precinct, drawing people together for worship at a time when doing so carried real legal risk. The carved initials, rendered in a lettering style borrowed from funerary stonecutting, give the boulder a faint solemnity, as though whoever commissioned or cut them wanted the rock to carry at least some of the dignity of a formal religious object.