That alone would make this a curious site. What compounds the curiosity is that nobody is quite sure what it actually is. A cist is typically a simple stone box used for individual burial, but the decoration here, along with the monument's original form, has kept scholars arguing for decades about whether something more complex is going on.

When the structure was recorded by the antiquarian Wood-Martin in 1887 and 1888, he noted that it had once been roofed by a single large slab, covered over with a mound, and enclosed within a stone circle. That fuller picture had already been partially destroyed by 1830, when ploughing disturbed the site. The ploughing turned up a paved stone floor, cremated human bone, and a cinerary urn, an urn used to hold the remains of the dead after cremation. What survives today is a roofless chamber measuring roughly 2.72 metres long and up to 1.20 metres wide, set into a south-south-westerly slope with open views in every direction, and aligned approximately east to west, open at the eastern end. One of the decorated slabs has since been removed. The combination of carved decoration, a surrounding stone circle, and a covering mound led Seán Ó Nualláin, writing in 1989, to list it as a possible passage tomb, a type of megalithic monument in which a narrow stone corridor leads to a burial chamber, often decorated in just this way. Others, including Evans, Herity, and Shee Twohig, have engaged with the question without reaching consensus, leaving Knocknashammer in an unresolved category of its own.