It is not a bridge, not a mill, and not a lock, but a regulating weir, a structure built to control and divide the flow of water, splitting the Dublin City Watercourse into two separate channels. That a piece of urban water infrastructure should earn such a vernacular nickname speaks to how familiar and locally significant it once was, even if the name has long since dropped out of common use.

The Dublin City Watercourse was a managed channel that carried water through the south city, and this particular stone-built feature sat at a critical junction within that system. According to the historian Weston St John Joyce, writing in 1912, the weir divided the watercourse into one branch running towards the Grand Canal Basin and another heading towards Dean Street. In practical terms, it was a piece of hydraulic engineering that gave the city some degree of control over where its water went, a modest but genuinely important function in a city that depended on managed water supply for mills, tanning, brewing, and domestic use. The 1837 OS map, one of the most detailed surveys of Irish urban geography produced in the nineteenth century, recorded it carefully enough to give it a name, which suggests it was a recognised landmark at the time rather than an anonymous culvert.

Much of the Dublin City Watercourse now runs underground or has been absorbed into later drainage works, so tracing any surviving physical evidence of the weir requires some patience and a willingness to read the landscape against historic maps. The Dean Street and Grand Canal Basin area of the south inner city is the place to focus attention. Overlaying the 1837 OS six-inch map, freely available through the OSi historical mapping portal, onto a modern street map can help orient a visitor to where the channel once ran. What you are unlikely to find is the weir itself in any obvious above-ground form, but the general lie of the land and the alignment of older laneways and boundaries sometimes preserves a ghost of the original watercourse. Joyce's 1912 work, 'The Neighbourhood of Dublin', remains a useful companion for anyone piecing together this particular corner of the city's water history.