A cist, in the language of prehistoric archaeology, is essentially a box burial, a chamber built from upright slabs to contain human remains, then covered and sealed. This particular example measures approximately 1.2 metres north to south and 1 metre east to west internally, modest dimensions that speak to a single interment rather than a collective tomb.

The chamber is lined with upright limestone flags: two sidestones along the eastern wall, one along the western, and a notably larger endstone closing the southern end, that southern slab measuring 1.4 metres in length and 0.12 metres thick. The gaps between the upright stones are packed with horizontally laid slabs, and above these, smaller cairn stones have accumulated over time. What makes the arrangement at Poulnaskagh particularly interesting is the partial kerb that borders the eastern side of the cist. Six non-contiguous kerbstones curve along that eastern flank at a distance of roughly 0.85 to 1.2 metres from the cist walls. Were this curving line to continue around the western side, it would form an oval kerb with an estimated diameter of around 3.6 to 4 metres, a kind of inner ring monument within the larger burial cairn. Whether that outer arc was ever completed, or whether only the eastern portion was ever laid, is not currently known.

The cist sits within its parent cairn rather than standing exposed, which means the full geometry of the site is not immediately legible from ground level. The interplay between the inner cist, the partial kerb surrounding it, and the broader cairn suggests a structured funerary landscape where different elements may have been added or emphasised at different moments during the site's use.