Twenty metres across and two metres high, it is built from horizontally laid slabs, a construction method that sets it apart from the rougher field-clearance cairns sometimes mistaken for prehistoric monuments. The surrounding bog has crept up around its base over millennia, which makes the structure feel both ancient and actively disappearing, caught in a slow process of absorption by the upland landscape.

The cairn has been tentatively identified by the archaeologist Michael Herity, writing in 1974, as a passage tomb. Passage tombs, for those unfamiliar with the term, are Neolithic megalithic structures typically consisting of a burial chamber reached by a narrow stone-lined passage, all covered by a mound or cairn. They were built by farming communities in Ireland roughly five thousand years ago and are most famously represented by monuments such as those in the Boyne Valley, though examples occur across a much wider range of upland and coastal locations. The Keadeen cairn sits just to the north of the mountain's summit, positioned with what the available evidence describes as very extensive views, a placement consistent with the known tendency of passage tomb builders to favour prominent, elevated ground. Whether the interior structure of a passage and chamber survives beneath the cairn material is not recorded.