Historical records place this royal stronghold in use from the mid to late 14th century, when the Mac Dermots held sway over this region of Connacht. The island itself, roughly 340 to 390 metres in diameter, occupies a strategic position at a narrow point in the lough, with the eastern and southern shores lying just 180 and 100 metres away respectively.

The main feature of the site is a rectangular grass-covered platform measuring 30 metres east to west and 29 metres north to south, surrounded by substantial flat-bottomed moats. These defensive ditches, between 4.8 and 5.3 metres wide at the top and narrowing to 3.2 to 3.5 metres at the base, would have presented a formidable barrier to any unwelcome visitors. The moats reach depths of up to 1.3 metres on the interior side and 0.9 metres on the exterior, with overgrown earthen field banks now marking their outer edges.

At the southwest corner, a rectangular annexe extends from the main enclosure, measuring 26 metres northwest to southeast and 22 metres northeast to southwest. This additional fortified area, defined by scarps and fosses, features its own defensive bank and ditch system along the southeastern side, with the bank standing 0.6 metres high on its outer face. A larger enclosure connects to the northeastern section of the site, suggesting this was once a complex of interconnected defensive structures that reflected the power and status of its royal occupants.