It is not a constructed altar in any conventional sense, but a ledge formed by outcropping rock that rises to about two metres, with the shelf itself standing at roughly seventy centimetres from the ground. That modest surface, sheltered and secluded, was once enough.

During the Penal era, roughly the late seventeenth and eighteenth centuries, Catholic worship was suppressed under a series of laws that banned the public celebration of Mass and barred priests from practising openly. In response, congregations throughout Ireland gathered at outdoor sites, often remote or naturally concealed, where a priest could conduct the liturgy with some protection from discovery. These locations are known as mass-rocks, and the Derryrush example follows the pattern well: the south-west aspect would have offered a degree of shelter, and the enclosing rock face would have provided both a focal point and a natural screen. Local tradition holds that Mass was indeed celebrated here during that period, and the landscape itself seems to confirm why this particular shelf was chosen over any more exposed spot nearby.

About ten metres to the north-west, on the opposite side of the Lehid-Kilmakilloge road, there is a possible bullaun stone. Bullauns are boulders or outcrops bearing one or more circular cup-shaped hollows, and they are found across Ireland in association with early Christian sites and holy wells, though their precise function remains debated. Whether its presence here is connected to the mass-rock or represents an entirely separate layer of local significance is not known, but the two features together give this small corner of south Kerry an accumulated quiet weight that repays attention.