It is, in fact, a fish weir dating to somewhere between 794 and 556 BC, a working piece of Iron Age infrastructure that has survived in the riverbank for the better part of three millennia. Fish weirs of this type, sometimes called fish traps or stake weirs, were built to intercept fish as they moved with the current, channelling them into a confined space where they could be caught easily. What makes this one arresting is not just its age but the legibility of its construction, still visible in the mud after all this time.

The weir runs for 32 metres along the bank, formed from two parallel rows of upright stakes set less than a metre apart. Horizontal rods, each between two and three centimetres in diameter and up to three metres in length, are woven around these uprights in a technique closely related to wattle construction, the same basic basketwork method used in building walls and hurdles throughout prehistoric Ireland. The uprights themselves vary from three to ten centimetres in diameter, with most clustering around five centimetres. The structure is broadly linear but bends into an S-shape at its northern end, and two small wattle panels protrude separately from the mud a little further to the west. Many of the uprights now lean eastward at roughly 45 degrees, a consequence of gradual bank erosion and the slow downslope movement of silt toward the water; stakes located further back from the erosion front remain closer to vertical, which helps confirm the lean is a product of time and ground movement rather than original design. The site was recorded and described by O'Sullivan in 2010, with the calibrated radiocarbon date placing its construction firmly in the late Bronze Age or early Iron Age.