Locally it is called Eochy's Cairn, and tradition holds that it marks the grave of King Eochai, said to be the last king of the Fir Bolg, the mythological people displaced by the Tuatha Dé Danann at the First Battle of Mag Tuired. Whether or not any historical figure underlies the legend, the monument itself is real enough: an oval cairn measuring some 43 metres east to west and rising to a height of 6 metres, with kerbing, the edging stones that define the cairn's boundary, still visible along its southern side.

The cairn sits within a henge, a type of prehistoric ceremonial enclosure typically defined by a bank and internal ditch, which already suggests a place of some ritual significance before or alongside the cairn's construction. Peter Harbison, writing in 1970, proposed that the mound may conceal a passage grave beneath it, one of the megalithic tomb forms common in Ireland during the Neolithic period, in which a stone-lined corridor leads to a central burial chamber. If he is right, the cairn at Carn would belong to a tradition of monumental funerary architecture stretching back perhaps five thousand years, long predating the Iron Age world of the Fir Bolg legend that later attached itself to the site. The monument is a National Monument in State guardianship, designated as No. 246, which affords it a degree of formal protection under Irish law.