In 1293, Thomas de Pykering, keeper of Athlone Castle, accounted to the exchequer for £4 16s. raised from the sale of 3,600 eels taken at the town. No physical trace of the weirs that caught them has been positively identified, but the paper trail they left across centuries of royal documentation is remarkably precise.

The weirs were originally built by the monks of St Peter and Paul's Priory in the late twelfth or early thirteenth century, most likely along the western bank of the Shannon, close to both the priory and the Anglo-Norman castle. A fish weir, in its simplest form, is a fixed barrier or enclosure set into a river to trap fish as they move with the current, and the Shannon's eel runs made such structures exceptionally profitable. The Anglo-Norman settlement of Athlone brought the fisheries into Crown possession early in the thirteenth century, and what followed was a long, often fractious negotiation between the monks and successive royal administrations. In 1216, King John was already instructing his justiciar to arrange compensation for the Prior, whose lands and fisheries had been disrupted. By 1284, King Edward I had to intervene again after the construction of two royal watermills on the monks' pool had left the priory taking nothing from the water at all. A payment of 40 shillings was eventually made to a monk named Gilbert in 1290 to cover three years of losses caused by those same mills. The Crown then leased the fisheries back to the priory at a rent, receiving £15 6s. 8d. in 1286 for the pools and river tolls combined. By the sixteenth century the weirs had passed through a succession of Crown leases to figures such as Andrew Brereton and Edmund O'Fallone, a local merchant, with conditions specifying that the President of Connacht could use the weir two nights each month, with nets, boats, and poles supplied by the lessee. A 1584 lease to George Alexander bundled seven weirs in the Shannon into a package that also included the priory site and surrounding demesne lands. The end came quietly during the Shannon navigation works of 1841 to 1849, when nineteen eel weirs were removed from the river at Athlone to allow for improved passage of boats.