Fish weirs work on a simple principle: stakes and woven fencing are arranged to guide fish, as the tide falls, into a narrowing enclosure or funnel from which they cannot easily escape. This particular structure, known in survey records as Boarland Rock 8, sits roughly 150 metres north of a related weir feature and forms a curved line some 37 metres in length, with a shorter secondary line of about 7 metres joining it near its southern end. Notably, no funnel survives or is visible, and a gap of roughly 2 metres interrupts the main line at around its midpoint.

What makes the structure quietly remarkable is its age, confirmed through radiocarbon dating of the surviving wood. The timber has been dated to approximately Cal. AD 1297 to 1398, placing its construction or use somewhere in the late medieval period, during the fourteenth century. This was a time when the estuaries and tidal rivers of Ireland's western coast supported extensive inshore fisheries, often managed by monastic communities or local lords. The Fergus Estuary, opening into the Shannon, would have been a productive stretch of water, and the presence of multiple weir features in this area, including a further fence exposed during fieldwork in September 2009 alongside a neighbouring structure, suggests something more organised than an isolated effort. The estuary appears to have been worked with some deliberateness, its shifting sands and tidal rhythms folded into the routines of medieval food production.