Measuring roughly 2.6 metres along its northwest to southeast axis and standing about 0.8 metres high, the stone is earth-fast, meaning it is fixed in the ground rather than placed there by human hands. Its roughly rectangular plan and the way its surface tilts gently from the southeast end down to a flatter northwest half made it, according to local tradition, well suited to a very particular purpose.

The boulder is remembered as a mass-rock, a category of site that speaks directly to one of the more austere periods in Irish Catholic life. During the Penal Laws, broadly in force through the late seventeenth and eighteenth centuries, the public practice of Catholicism was suppressed, and priests caught celebrating Mass risked severe punishment. Communities responded by gathering in remote or inconspicuous locations outdoors, using whatever the landscape offered as a makeshift altar. A flat-topped boulder in open bog, away from roads and settlements, was exactly the kind of place that served. The priest would stand at the stone, the congregation behind him on the hillside, with the surrounding terrain offering both concealment and a clear view of anyone approaching. The boulder at Caherkeen fits this pattern: exposed enough for a gathered crowd, elevated enough to carry a view of the bay below, and formed by nothing more than geology into something close to functional.