One is an irregular upright block, aligned roughly north to south and standing 1.8 metres tall; the other, a large prostrate slab lying about 1.7 metres to the west, measures over two metres in length. The grouping is officially classified as anomalous, meaning surveyors have not been able to assign it confidently to any recognised monument type. That uncertainty is part of what makes the site quietly compelling.

What gives the stones their most interesting dimension is local tradition rather than archaeology. It was supposed, at least by the time Bowman recorded it in 1934, that Mass had been said here during the Penal times. The Penal Laws, which were in force in various forms from the late seventeenth century through much of the eighteenth, placed severe legal restrictions on Catholic worship in Ireland, effectively banning priests from ministering openly. In response, outdoor gatherings around natural or pre-existing landmarks became a practical solution, and these sites are known in Irish tradition as Mass rocks. Whether the stones at Marybrook were originally set up for religious use, or were already standing in the landscape and simply pressed into service, is not recorded. The prosrate slab in particular would have served well as a makeshift altar surface, flat and broad enough to hold the vessels required for the celebration of Mass. The OS Memoranda reference suggests the tradition was noted by Ordnance Survey fieldworkers in the nineteenth century, giving it some antiquity as local knowledge even if the original purpose of the stones themselves remains unresolved.