What was once a structure 47 metres long has been cut nearly in half, its western end converted into a livestock underpass during a road-widening scheme in 1985 and 1986. The remaining 23.5 metres still stand, and the quality of the surviving stonework makes the loss of the rest all the more striking.

The lock at Longueville was the second of two on a canal drawn from the River Blackwater. The first, Pallas lock, no longer survives at all. At Longueville, the retaining side walls, built of coursed ashlar limestone, that is, stone cut into regular blocks and laid in neat horizontal courses, rise to around four metres and span a width of just over six and a half metres. At the eastern end, a vertical niche cut into the stonework once held a lock gate, the mechanism by which water levels were raised or lowered to allow boats to pass. That niche is still visible, a precise absence in the masonry that speaks to the lock's original working purpose. The canal itself was taken from the Blackwater at its western end, connecting the river to whatever commercial or agricultural traffic once moved through this stretch of North Cork.

The structure today is a hybrid of two different histories: one of careful Georgian-era engineering, the other of mid-1980s rural pragmatism. The road widening that claimed the western end was not vandalism in any meaningful sense, simply the ordinary pressure of modern agricultural life imposed on an old waterway. What remains is the eastern section, its limestone walls intact, the gate niche preserved, and the ghost of the full lock length readable in the abrupt termination at the western end.