Standing around 0.8 metres high and roughly a metre wide, it is an undressed stone, shaped by nothing more than time and weather, with a single small field stone placed at its south-western base. That modest arrangement is the only outward sign that this boulder once served as an altar.

Mass-rocks are closely associated with the Penal Laws, the body of legislation enacted against Catholics in Ireland from the late seventeenth century onwards, which at various points prohibited Catholic worship, the building of churches, and the public ministry of priests. Communities adapted by gathering outdoors, often in remote or inconspicuous locations, where a flat-topped stone or boulder would serve as a makeshift altar. The priest would celebrate Mass in the open air, with lookouts posted against the possibility of discovery. The Esker boulder fits this pattern precisely: unworked, unassuming, and set into ordinary farmland in a part of Connacht where Catholic observance was driven into the landscape itself. The small field stone at its base may have served a practical purpose, perhaps as a kneeling stone or a support, though its exact original function is not recorded.