A weir, in this context, is a low barrier built across a river to control the flow of water and, very commonly in medieval Ireland, to trap fish. Nothing survives at the precise spot the map indicates, but aerial photography taken between 2011 and 2013 reveals a stone weir sitting about 80 metres to the north, suggesting the feature has shifted in the cartographic record, or perhaps that the 1841 surveyors were marking a memory rather than a structure still clearly legible on the ground.

The weir's significance deepens considerably when set against a legal document from 1303 to 1304. The Plea Roll of 32 Edward I records a dower claim brought by a woman named Sibilla, widow of Maurice son of Thomas, against Nicholas son of Maurice. Sibilla was seeking her rightful third share of a house, a watermill, over a thousand acres of land, and, crucially, a fishery on the River Feale, all within the vill recorded as Lystothyl, which is Listowel. The fishery named in that document is thought to correspond to this weir site. Listowel Castle and a mill site both lie roughly 385 metres to the north, and that cluster of medieval infrastructure, castle, mill, fishery, suggests a small but organised economic landscape along the Feale in the early fourteenth century. A second structure recorded as 'Old Weir' lies 230 metres to the southeast in the neighbouring townland of Ballygrenane, hinting that this stretch of river was managed and exploited at multiple points simultaneously.