It is a massive mound of loose stones, roughly 55 metres in diameter and around 10 metres high, and it has never been formally dug into by archaeologists. That fact alone gives it a peculiar quality among prehistoric monuments: whatever lies beneath, if anything does, remains entirely unknown.

The cairn is traditionally associated with Méabh, the legendary queen of Connacht from the Ulster Cycle of Irish mythology, and is commonly called Miosgán Méabha, sometimes rendered in English as Maeve's Lump or Maeve's Cairn. Whether the monument has any genuine connection to a historical figure is another matter. The structure itself is Neolithic in character, broadly comparable in type and probable date to the great passage tombs of the Boyne Valley, and is thought to belong to a period roughly five thousand years ago. Knocknarea as a whole sits within a wider prehistoric landscape that includes numerous other monuments on and around the hill, suggesting the area held sustained ceremonial significance over a long period. W. B. Yeats, who grew up nearby in Sligo, was drawn to the hill and its associations; he referenced Méabh in his poetry, cementing a Romantic literary layer over what is already a complex archaeological one.

The summit is reached by a well-worn path, and visitors are traditionally asked to carry a stone up and add it to the cairn rather than remove one, a custom that has its own quiet logic given how much of the monument has been disturbed over the centuries by people doing exactly the opposite. The approach takes roughly forty minutes from the car park at Knocknarea village, and the cairn itself comes into view only near the top, appearing suddenly large once you reach the plateau.