That uncertainty is itself part of what makes the site worth knowing about. A massive irregular capstone, roughly 4.6 metres by 4.3 metres, lies across two supporting stones, one upright and leaning slightly inward, the other horizontal and resting on a bed of smaller stones. A further two stones, each over a metre and a half in length, lie stacked one above the other about two metres to the west. The whole arrangement is substantial enough to demand attention, yet ambiguous enough to have divided scholarly opinion for decades.

The debate has a paper trail. In 1945, the archaeologist M. J. O'Kelly, who would later become famous for his excavations at Newgrange, proposed that the structure was the ruined remains of a wedge-shaped gallery grave, a type of Neolithic or Early Bronze Age tomb common in the west and south of Ireland, typically consisting of a long, slightly tapering stone chamber covered by a cairn. It is a plausible reading of what is on the ground. However, when Ruaidhrí de Valera and Seán Ó Nualláin compiled their systematic survey of Irish megalithic tombs in 1982, they judged the Castlemary feature too doubtful to include in their main catalogue. The stones are real, the scale is undeniable, but whether what remains represents deliberate prehistoric construction or a later disturbance, or something else entirely, has never been settled.