Beneath it rises a conical limestone cairn, its base spanning 27 metres, its flat top measuring 8 to 9 metres across, and its height reaching 7.5 metres. The cross is visible enough, but the mound it crowns is the stranger object, a prehistoric structure that has resisted easy classification for decades.

Passage tombs are among the oldest monumental forms in Ireland, typically consisting of a stone-lined burial corridor leading to a chamber, all covered by a large circular mound. At Sheemore, however, archaeologists Ruaidhrí de Valera and Seán Ó Nualláin, writing in their 1972 survey of megalithic tombs, noted that no kerb stones edging the mound's base and no internal chamber have been identified. Despite this absence of the usual diagnostic features, they considered it probable that the cairn is indeed a passage tomb. That tentative classification has remained the working consensus. The monument is protected under a preservation order dating from 1978, which at least means the uncertainty has not been resolved by any further disturbance to the mound itself.

The combination of a prehistoric cairn and a mid-twentieth century devotional cross is not unique in Ireland, where hilltop monuments have attracted successive generations of meaning-making, but Sheemore's version is unusually stark. The 1950 cross sits on a Neolithic structure that may be five thousand years old, and the two objects simply coexist, each ignoring the other's logic entirely.