The likely culprit for that fracture is a tree, its roots pushing down through accumulated peat directly onto the capstone over what may be thousands of years. It is a quietly telling detail: nature working away at a structure that predates written history, in a clearing within a plantation that would otherwise swallow any sense of the place entirely.

This is a wedge tomb, a type of megalithic burial monument built during the late Neolithic and into the early Bronze Age, broadly between 2500 and 2000 BC. Wedge tombs take their name from the characteristic shape of their galleries, which are typically higher and wider at the entrance end and taper inward. The Aghnacally example follows this form closely. Its gallery runs four metres in length, open at the south-west end, where it is both taller and broader. Two large upright stones, or orthostats, form the northern wall of the gallery; three form the southern side. The backstone is placed slightly beyond the ends of those side walls rather than flush with them, a detail recorded by the archaeologists Ruaidhrí de Valera and Seán Ó Nualláin in their 1972 survey of megalithic tombs across Ireland. Beyond the gallery itself, there are outer walling features: four stones running along the north side, and two concentric lines of walling to the south, the inner comprising four stones and the outer three. The whole structure sits within an earthen mound measuring nine metres east to west and seven and a half metres north to south, giving the monument a modest but definite presence in the landscape even after millennia of settling and overgrowth.