The two are separated by about 70 metres, an arrangement that raises more questions than it answers about who built them, and why they chose to live surrounded by water rather than on the shore nearby.

A crannog is an artificial or partly artificial island, typically constructed during the early medieval period in Ireland and Scotland, though some examples are considerably older. Inhabitants drove timber piles into the lakebed and built up a platform of stone, brushwood, and earth, creating a dwelling that was defensible by virtue of its position alone. The example in Barnagrow Lough came properly into view only after water levels in the lough were lowered around the turn of the twentieth century. When the scholar Hall examined it in 1910, he found a raised central mound about 20 metres in diameter, ringed by a substantial number of stakes projecting outward from the edges, presumably part of the original defensive or structural framework. On the summit, excavation or survey turned up the possible foundations of a small house, a broad hearth stone, and four quernstones, the paired grinding stones used to mill grain, though notably none of the four matched one another, suggesting they had arrived on the island piecemeal, perhaps replaced or salvaged over a long period of use.