The smaller, later structure sitting atop the earlier mass is almost certainly a nineteenth-century trigonometrical station, the kind of survey marker the Ordnance Survey planted across Ireland's high ground when mapping the country in the 1840s. Beneath it, however, is something considerably older and more layered: a roughly circular prehistoric cairn, nearly 20 metres across, built of limestone stones and slabs, and constructed in what appear to have been several distinct phases across a very long span of time.

The prehistoric cairn is thought to date originally to the Neolithic or Bronze Age, but its present form reflects repeated modification. Archaeologists working on the structure in 2017 identified a series of internal revetments, retaining walls of drystone construction that create a stepped or terraced profile rising to a height of around 2.5 metres. Each terrace may represent what researchers call a "wrapping event": a deliberate enlargement of the monument, probably to incorporate new burials, with the older cairn effectively enclosed within each new layer of stone. A similar pattern has been documented at the Poulawack cairn nearby in the Burren. Much of the outer edge of the cairn has been obscured by slippage over the centuries, though remnants of a kerb, the boundary stones that would originally have defined the monument's edge, are still visible along its western and southern sides. Both elements of the double cairn underwent conservation and consolidation works in 2017. When the antiquarian Thomas Westropp visited in 1901, he recorded it as nearly levelled; the structure that remains is a good deal more substantial than that description might suggest.

The name adds another dimension. The cairn appears as "Doughbranneen" on Ordnance Survey maps from 1842 and 1915, and as "Dobhach Bhrainín" on Tim Robinson's celebrated map of the Burren from 1977. About 470 metres to the north-north-east, at a lower elevation, sits a second cairn known as Carnsefein, from the Irish "Carn Suí Finn", meaning the cairn of Finn's seat. Taken together, the two names point toward the Fianna, the legendary warrior band led by Fionn mac Cumhaill, whose mythological geography was far from merely decorative. Scholars have argued that such place-name associations played a meaningful role in medieval political discourse, lending a landscape a kind of heroic legitimacy. On Carnseefin, overlooking Galway Bay, the layers of meaning are almost as compressed as the layers of stone.