No trace of the mills survives above ground, and their precise locations have not been identified, yet the documentary and cartographic record is unusually detailed. John Speed's 1610 map of Dublin City shows two watermills clearly enough that their waterwheels, fixed to the gables of buildings on the western side of the watercourse, are individually depicted. A third mill is tentatively identified as the southernmost building on the same map, straddling the stream just to the west of John's House, in the area north of Thomas Street.

The mills, collectively known as St John's Mills, belonged to the Hospital of Saint John the Baptist, a medieval religious house and hospital situated outside the New Gate on the south-western edge of the walled city. The watercourse that powered them was drawn from the Odder and ran east to west along what is now Thomas Street before turning north, likely along the line of John's Lane West, and eventually reaching the Liffey somewhere near the vicinity of present-day Usher's Quay. The city's relationship with this water supply was carefully managed and occasionally fractious. In 1451 the Dublin Assembly Roll authorised a James Dowdall to offset his repair costs against the annual rent he owed for use of the water running to the mill of St John's. In 1457 the Assembly granted the watercourse back to Saint John's House on the condition they pay forty shillings yearly, with the pointed proviso that if they failed to pay, a John Staines, plumber, would have the water for forty years. A more formal forty-year lease followed in 1458, recorded in the Dublin White Book, which set out the full route of the watercourse from a small building called le watyrhouse near Crokkers Gate, through the King's street of Saint Thomas, and on to the hospital mill. The lease also permitted the Prior to dig a fosse from the mill to the city ditch so the water could drain freely to the Liffey, and reserved the right of the Mayor and commonalty to divert the flow to fill the city ditch in times of need. After the Dissolution of the Monasteries, the church of St John's was demolished and its materials sold off, but when Edmund Redman was granted the site in 1539, the three watermills were explicitly excluded from his lease, suggesting they retained practical and commercial value. By 1564 the Assembly had granted a Christopher Sedgrave the watercourse for sixty-one years at the same annual rent of forty shillings.

A section of the watercourse was excavated by archaeologist Linzi Simpson beneath the north-eastern corner of the National College of Art and Design on the south side of Oliver Bond Street, which gives some physical anchor to what is otherwise a largely invisible landscape. Speed's 1610 map remains the most useful guide to the general layout, showing the watercourse labelled as number 51 running past the northern side of St Catherine's Parish Church on Thomas Street before bending northward toward the mill complex. Rocque's 1756 map of Dublin also shows the area, including Mullinahack Lane to the north of John's House. Neither map nor excavation resolves the exact position of all three mills, and the area today offers no obvious surface signs of what once stood there, making old maps the most rewarding tool for anyone wanting to trace the ghost of this industrial waterscape through the modern streetplan.