It is not the grandest megalithic monument in Ireland, not even close, but its dimensions tell an oddly specific story. The cairn measures roughly 9.5 metres across, edged by a kerb of small boulders, and at its centre lies a rectangular chamber so compact, at 1.2 metres long, 0.75 metres wide, and only 0.6 metres high, that no living adult could do more than crouch inside it. A single roofstone, 1.8 metres by one metre and about 20 centimetres thick, partially covers the chamber still. A short passage once ran from the kerb inward from the west-south-west side, the characteristic feature that places this firmly in the passage tomb tradition.

Passage tombs are among the oldest monument types in Ireland, generally dated to the Neolithic period, roughly 4000 to 3000 BC, and typically consist of a stone-lined burial chamber reached by a narrow stone-lined corridor, the whole thing buried under a mound of earth or rubble. The Lackan example is modest by comparison with famous sites to the north, but its upland position follows a pattern well recognised in the archaeology of the Irish passage tomb tradition. The site was noted by the archaeologist Michael Herity in his 1974 survey of Irish passage tombs, which remains a foundational reference for monuments of this type. The possible recess opening off the chamber, if it is indeed structural rather than accidental loss, would add a further layer of complexity to an already intriguing little structure.