Unremarkable by appearance, it carries a local designation that locates it within one of the more quietly charged chapters of Irish history: it is known as a mass-rock.

Mass-rocks were the improvised altars used by Catholic priests during the Penal Law era, roughly the late seventeenth and eighteenth centuries, when the open practice of Catholicism was suppressed under English colonial legislation. Congregations gathered in remote townlands, on hillsides, in valleys, at boulders like this one, to hear Mass away from the scrutiny of authorities. The boulder at Killary sits in a natural setting that would have served that purpose well, tucked alongside a north-south running stream in a mountain valley, sheltered by a field boundary. The stone itself measures 0.8 metres high, 1 metre wide, and 0.4 metres thick, substantial enough to serve as a flat surface for the altar vessels a priest would have carried with him.