It is the remains of a mill-flume, the enclosed trough used to direct water onto the wheel of a horizontal-wheeled mill, and it dates to around 827 AD.

Horizontal-wheeled mills, sometimes called Norse mills, were the most common type of water mill in early medieval Ireland. Unlike the familiar vertical waterwheel, they used a horizontal wheel set directly beneath the millstone, driven by a jet of water channelled through a narrow flume. The Kilphillibeen example was found on the western side of a stream, and its oak timbers survived in good enough condition to be dated by dendrochronology, a technique that establishes age by analysing the pattern of annual growth rings in wood. The flume measures 3.32 metres in length and 0.8 metres in width, dimensions that place it comfortably within the range known from comparable early medieval sites elsewhere in Ireland. The dating, to approximately 827 AD, puts its construction in the middle of the Viking Age, when such mills were a fixture of monastic and secular settlements across the country.