Alongside the weir's timber posts sits a double basket trap, woven from hazel and alder rods less than a centimetre in diameter, and still sufficiently intact after seven or eight centuries to reveal exactly how it worked. A fish weir is a fixed barrier or enclosure built into a waterway to intercept fish as the tide recedes, and this example, sitting in the middle of a tidal estuary, would have funnelled its catch with quiet efficiency.

Radiocarbon dating of wood sampled from the weir places its construction somewhere between AD 1224 and 1291, during a period when tidal fisheries of this kind were a significant source of food and income along Ireland's estuaries. The weir itself consists of one line of posts roughly nine metres in length and a shorter line of just a couple of metres, oriented north to south. Its uprights are notably slender, only three centimetres in diameter, finer than those of the nearby Boarland Rock weirs. The double basket trap at the southern end is where the real craft lies. The two baskets served different functions: the outer one, Basket 1, was made predominantly from hazel and acted as the entry point, guiding fish inward; the inner one, Basket 2, constructed mostly or entirely from alder with slimmer stakes and strands, was where the fish were held. The alder and hazel used throughout were cut in autumn or winter, when the sap is low and the wood most suitable for the close, flexible weaving that basket-making requires, though the timber could have been stored and re-soaked before use. Boarland Rock 4 sits within a cluster of weirs in the same estuary, suggesting this stretch of the Fergus was worked intensively and methodically by communities who understood its tides intimately.