The stone is a glacial erratic, a rock deposited far from its geological origin by retreating ice sheets and left stranded in the landscape thousands of years later. This one measures roughly three metres north to south, two metres east to west, and stands about one and a half metres high. According to local tradition, it served as a mass-rock during the Penal era, a makeshift altar at which Catholic priests celebrated the Mass in secret when doing so openly was a criminal act.

The Penal Laws, which were enforced with varying degrees of severity across the seventeenth and eighteenth centuries, banned Catholic worship and drove priests into the countryside, where congregations gathered at remote or inconspicuous spots. A flat-topped or naturally sheltered stone could serve as an altar, and these sites, scattered across rural Ireland, became known as mass-rocks. They were chosen for practical reasons: difficult to observe from a distance, easy to abandon quickly, and leaving no permanent structure that authorities might demolish. The boulder at Caher fits that pattern well enough, standing in open but quiet ground beside a river, away from roads and settlements.