It is the displaced capstone of a prehistoric cist, a stone-lined grave box of a type built during the Bronze Age, and its separation from the burial it once sealed is one of those small, quietly unsettling details that accumulates meaning the longer you stand looking at it.

The cist itself is roughly square in plan, defined by granite boulders and measuring approximately 2.1 metres north to south and 1.4 metres east to west, with a depth of around 0.7 metres. The displaced capstone, which measures 2.1 metres long and 1.4 metres wide, now rests to the west of the cist. Cists of this kind were typically used for individual or small-group inhumations or cremations, the body placed in the stone-lined box and the whole structure then covered by a earthen or rubble mound. At Lugmore, traces of that original covering mound are still legible in the landscape, with a diameter of approximately 9 metres and a surviving height of around 1 metre. This information was recorded by Healy in 1974 and compiled by Geraldine Stout as part of the broader archaeological survey work that has documented Dublin's upland monuments.

Tallaght Hill sits within the northern Dublin Mountains, and reaching the summit requires some effort on foot across open upland terrain. The site is exposed and the ground can be wet, so appropriate footwear matters. There are no interpretive signs or formal markers at the cist itself, so it rewards visitors who arrive with the site dimensions and location noted in advance. The mound traces are subtle at ground level and more legible when the light is low and raking, which makes early morning or late afternoon visits in autumn or winter particularly useful for reading the earthwork. The capstone, large and obvious once you know to look for it, lies just west of the stone box it once covered, and the gap between the two is the most immediately arresting thing about the place.