It is a fish weir, or more precisely a fish trap, a technology so old and so practical that versions of it appear on coastlines across the world, yet individual examples are easily missed, easily forgotten, and rarely recorded before the sea or simply time finishes them off.

This particular trap measures roughly 7.5 metres east to west and about 3 metres north to south, its walls built from local stone and oriented to make use of the tidal movement between the lough and the bay. The principle is straightforward: fish follow the retreating tide into a walled enclosure and cannot find their way back out as the water drops, leaving them stranded and catchable. What is quietly remarkable about the Keerhaun example is that it was in use until relatively recently, according to local knowledge, meaning it belongs not only to the distant past but to living memory of working the shoreline in this part of County Galway.