Its exact position has never been pinned down, which makes it an odd kind of historical presence: documented, referenced on an early map, and yet physically unlocated. It was one of four watermills that belonged to St John the Baptist's Hospital, a medieval religious house whose footprint is largely invisible to anyone walking the area today.

The hospital, known as Saint John's House, controlled a watercourse that fed at least three mills positioned to the west and north of the religious foundation. That watercourse appears on John Speed's 1610 map of Dublin City, drawn at a moment when the medieval street pattern was still largely intact. Speed's map shows the channel running along St Thomas Street, between St Catherine's Church and John's Lane West, with a label marking 'The mill's'. It is one of the more quietly remarkable details in that famous survey of the city. The watercourse itself was not merely a cartographic notation; in 1995, archaeologist Linzi Simpson excavated a section of it beneath the north-east corner of what is now the National College of Art and Design, on the south side of Oliver Bond Street, west of John Street. Finding a medieval water channel running beneath a working art college in the middle of Dublin is the kind of discovery that rarely makes headlines but says a great deal about what lies undisturbed beneath urban ground.

There is nothing to see at the surface. The area around Thomas Street, John's Lane West, and Oliver Bond Street is thoroughly built over, and the National College of Art and Design occupies the ground above the excavated channel. Speed's 1610 map, however, is widely reproduced and worth seeking out in detail; the section showing the watercourse and mill label gives a clear sense of how the medieval city organised its industry around managed water. The Royal Irish Academy and Dublin City Library both hold material relating to the archaeology of the Liberties, and the excavation record from Simpson's 1995 investigation would be the most direct route into the physical evidence of what once ran beneath these streets.