The grotto itself, tended by the Sisters of Charity, gives no particular indication that the ground beneath it was once engineered to carry fast-moving water to a working mill, yet excavations inside the convent gate have exposed exactly that: curved brickwork almost certainly associated with the channel that once fed the mill's wheel.

The watermill at Donnybrook has a recorded history stretching back to at least 1524, when it appears in documentary sources cited by the local historian F. E. Ball. A millrace is simply the artificial channel used to direct river water onto or under a mill wheel, and by 1701 a lease map, brought to light through research by Danny Parkinson, shows the mill sitting immediately south of Donnybrook Castle, with its millrace cutting south-west to reach the River Dodder. That map places the mill squarely where the convent now stands. Archaeological testing carried out in 2001 at 29 Eglinton Road, a short distance away, uncovered a section of a post-medieval millrace running some 60 metres in length and approximately 5 metres in width, which may form part of the same system noted by Ball and visible on the 1701 map, as recorded by Delaney in 2003.

The convent grounds are not publicly accessible in the ordinary way, so the brick arches and their context remain largely out of reach for casual visitors. The Eglinton Road testing site is now built over. What can be done is to walk the stretch of Eglinton Road near its junction with the older Donnybrook streetscape and consider the invisible infrastructure below, a channel wide enough to carry substantial flow, threading its way from the Dodder through what is now a quiet residential and religious precinct. The Dodder itself is never far, and following its bank here gives some sense of why mills clustered along it for centuries, the river dropping steadily enough to make it useful long before the southern suburbs of Dublin existed in any recognisable form.