An eel weir is a fixed structure built across a watercourse to trap migrating eels, typically using wicker or timber channels that funnel fish into baskets or nets. What makes these particular features quietly remarkable is the paper trail behind them, which leads back through the Reformation and into the medieval economy of a long-dissolved Augustinian priory.

The Augustinian priory of Tristernagh, sometimes called the Priory of Kilbixy and dedicated to the Blessed Virgin Mary, was founded in the late twelfth century by Geoffrey de Constantine of Cosmeston Castle in Glamorganshire. For centuries it stood near the shore of Lough Iron, a lake that has itself shifted considerably since a drainage scheme on the Inny River in the late 1960s moved its effective shoreline from around 280 metres northeast of the site to roughly 650 metres away. In 1540, when an extent of the priory lands was drawn up at Tristernagh, the document recorded a weir on the water then called the Nany Water, identified with the Inny River, described simply as the Prior's Weir and noted as yielding 180 eels annually. After the dissolution of the priory, the property passed through the crown and in 1609 King James I granted the entire complex, including a weir referred to in the grant as the Frier's Weir on the Naniwater in Kilbixy parish, to Adam Loftus of Rathfarnham Castle. The precise location of the medieval Prior's Weir on Lough Iron has never been pinned down, but the weirs that appear on the 1838 map, sitting on the river at the townland boundary, are thought to possibly occupy the same position as those mentioned in the 1540 extent. It is a reasonable inference rather than a certainty, but it places a modest nineteenth-century cartographic detail within a line of documented use stretching back at least five hundred years.