A weir of this kind worked by funnelling the current through narrow channels or traps, making it possible to intercept fish, particularly salmon, moving upriver. The site is easily overlooked today, but the water passing over it has been doing much the same work, in one form or another, for an extraordinarily long time.

The historian Went, writing in 1954, identified the Islandbridge area as the location of the first weir on the Liffey. For a significant period of the medieval era, the weir and its salmon fishery were held by the Hospital of St John of Jerusalem at Kilmainham, a house of the Knights Hospitaller, a military-religious order that also managed considerable estates across Ireland. When the hospital was dissolved during the sixteenth-century suppression of the monasteries, the weir was recorded in formal terms as possessing four hachis, a word for the trap or cage mechanisms set into the structure to intercept fish as they moved through. That bureaucratic inventory, dry as it is, offers a rare glimpse into the practical workings of a medieval fishery. The weir at Islandbridge is believed to occupy, or to closely follow, the line of that earlier structure, and it remained in active use until the eighteenth century.

The Islandbridge area sits to the west of Phoenix Park, along a stretch of the Liffey that feels quieter than the quays closer to the city centre. The weir itself is visible from the riverbank and from the nearby road bridge, and the surrounding area is accessible on foot. There is nothing to mark the medieval history of the structure directly, so the satisfaction here is one of inference and imagination rather than interpretation boards. The salmon that once drew a monastic order to maintain traps in the current are largely gone from this part of the river, but the physical logic of the place, a natural chokepoint in the river's flow, remains as legible as it ever was.