But this particular structure carries an older purpose behind it: the control and harvesting of fish, a practice documented at this stretch of the Liffey for several centuries at least. Fish weirs, which are barriers built across a river to trap or direct fish into nets or baskets as the water passes through, were once a significant economic asset, as valuable in estate records as tillage land or pasture.

The earliest documentary reference to a fishery at this location comes from the Civil Survey of 1654 to 1656, a detailed inventory of land ownership compiled in the aftermath of the Cromwellian conquest of Ireland. The survey records a fishery belonging to Luttrelstown, the estate that takes its name from the Luttrell family, whose holdings in this part of County Dublin were extensive. The connection between that recorded fishery and the weir that exists on the river today was drawn by the historian Arthur Went in 1954, who concluded that the present structure is in all likelihood the same site, or at least its direct successor, mentioned three centuries earlier in the Civil Survey.

Yellow Walls itself sits in an area of north County Dublin where the Liffey curves through relatively quiet terrain before the river's lower reaches become more heavily developed. The weir is most legible as a feature when river levels are lower, typically in drier months, when the stone construction becomes visible rather than simply audible. There is no dedicated visitor access or formal interpretation at the site, so it rewards the kind of attention you might give to an unmarked feature on an Ordnance Survey map, something noticed rather than announced. For anyone interested in the relationship between estate economies and river management in early modern Ireland, the brevity of the written record here is itself telling; a single line in a seventeenth-century survey, and a weir still sitting in the water.