It measures roughly six metres by six metres, defined by a single course of carefully laid stones, with traces of what may be a flagged floor still visible inside. For most of the tidal cycle, it is simply gone, swallowed by the sea. Only at low spring tides does it briefly reappear.

The structure was identified by archaeologist Michael Gibbons and Shane Joyce of Clifden, lying to the east of a small islet recorded as Islandagar on the Ordnance Survey six-inch maps. Approximately ninety metres to the north-west, a curving stone wall was also noted. The two features together point towards something larger and older than either structure alone: a drowned prehistoric landscape, a stretch of land that was once dry ground before rising sea levels gradually reclaimed it. Such submerged landscapes are found at various points around the Irish coast and represent some of the most tantalising territory in Irish archaeology, places where the ordinary evidence of how people lived, built, and moved has been preserved not in the ground but beneath the water.

Because the structure only surfaces during low spring tides, timing matters considerably. The islet of Islandagar provides a reference point for orientation, but the site itself is underwater for most of any given visit to the bay. Anyone hoping to observe it would need to plan carefully around tidal charts, and even then the window is narrow.