Fish weirs of this kind work by directing fish into a narrow funnel or trap as they swim with the current, making them an efficient and low-maintenance means of harvesting river fish without nets or rods. The structure itself is largely invisible to a casual visitor today, but its outline was still clear enough in 1840 to be recorded on the first edition Ordnance Survey six-inch map, sitting in the river immediately north of the mid-section of the bridge.

The weir has a paper trail that reaches back further still. The Civil Survey, a mid-seventeenth-century inventory of land ownership conducted in the aftermath of the Cromwellian wars, records a weir on what it calls the River of Shewer belonging to the Lord of Cahir, described as being near his mansion house, which is to say Cahir Castle itself. The quotation, preserved in Robert Simington's 1931 edition of the survey, ties the weir directly to the Butler lords who controlled the castle and the surrounding territory. For a medieval and early modern lord, a weir on a productive salmon river was not a minor amenity but a significant economic asset, and the phrasing of the survey suggests it was considered part of the castle's broader estate rather than a separate concern.