That administrative curiosity is the least strange thing about it. The structure runs for roughly 95 metres, sinuous rather than straight, built from large slabs, and at its eastern end it makes a sharp turn northwards before terminating in a cluster of substantial boulders, as if the whole thing was deliberately concluded rather than simply left off. Buried within the cairn, about two-thirds of the way along towards that eastern end, is what appears to be a megalithic chamber, a stone-built enclosure of roughly 5 metres by 2 metres, formed by upright slabs and aligned with the long axis of the cairn. No passage or entrance has been identified leading into it, which sets it apart from the more familiar passage tomb tradition and leaves its original function genuinely open.

Long cairns of this type belong broadly to the Neolithic period, and the category describes exactly what the name suggests: an elongated mound of stone or earth, sometimes covering burial chambers, sometimes containing structural features whose purpose remains unclear. What makes the Aghawinnaun example particularly unusual, according to research by Bergh published in 2008 and expanded in 2016, is its sinuous profile and the apparent deliberateness of that northward turn at the terminus. The western, uphill end has suffered some damage over time, with stones removed to build field walls in the surrounding landscape, a fate common to many prehistoric monuments across Ireland where dressed or conveniently sized stone was simply too useful to leave alone. The remainder, though, sits largely intact on a northeast-facing slope with open views across south Galway and northeast Clare, the cairn itself straddling the border as if indifferent to it.