This was a millrace, an artificial or managed watercourse designed to carry water from a river to drive the wheel of a mill, and the one associated with this site drew its flow eastward from the Camac river, a smaller tributary that enters the Liffey near Islandbridge.

What makes the history of this watercourse particularly interesting is its longevity on the historical record. It appears on John Speed's map of Dublin in 1610, one of the earliest detailed cartographic records of the city, which suggests the channel was already an established feature of the urban landscape by that point. By the time the first edition Ordnance Survey six-inch map was produced in 1838, the millrace was still clearly marked, running its eastward course alongside the Liffey. The mill it served had by the nineteenth century become known as Usher's flour mill, recorded as such by De Courcey in 1996, drawing on earlier sources. The Usher family were a prominent Dublin merchant family, and the Liberties and Coombe districts nearby were historically dense with industrial activity, including milling, weaving, and brewing, so a working flour mill in this vicinity would have been a practical anchor of the local economy.

The physical traces of the millrace are no longer easy to read in the landscape. Urban development across this part of the city has covered or redirected most of what was once a functional waterway, and a visitor today is more likely to encounter the area through maps and archival sources than through anything visible on the ground. The relevant stretch lies in the general vicinity of the Camac's confluence with the Liffey, and anyone with an interest in Dublin's industrial past would do well to consult the first edition OS six-inch map, which is freely available through the Ordnance Survey Ireland historical map viewer, to trace the original line of the race against the current street layout. The contrast between what was recorded in 1838 and what stands there now says a great deal about how thoroughly the working waterways of early modern Dublin have been absorbed into the city.