Look closer, and the remnants of a timber and wattle palisade are still intermittently visible where the stonework meets the water, best preserved on the south-western side. This is a crannog, an artificial or partly artificial island dwelling of the sort built and inhabited in Ireland from prehistoric times well into the medieval period, constructed from layers of stone, timber, brushwood, and peat, and typically ringed by a defensive palisade of the kind still faintly legible here.

The island measures roughly 20.5 metres north to south and 15.5 metres east to west, modest in scale but evidently significant enough to appear, at least by inference, in some of the more violent passages of late medieval Connacht history. The Annals of Loch Cé record that in 1345 four sons of Cathal Mac Raghnaill were executed on Loch an Scuir, and in 1390 O'Rourke escaped to the castle of Loch an Scuir, suggesting a fortified structure of some importance on or near the water. By 1580 the situation had grown bloodier still: Loch an Scuir was taken and Maelsechlainn Mac Raghnaill was slain there. Whether these annalistic references all point to the same crannog is uncertain, but the cluster of events around the one lough gives the island a particular weight. Adding to that sense of long occupation, a large disc quern used for grinding grain and a stone mould for casting a palstave, a type of early bronze axe-head, were also recovered from the site, as recorded by Wood-Martin in 1886, indicating activity here across a considerable span of time.