It is not a castle. What survives is a circular mound roughly a metre high, with a central rectangular hollow lined with stones, measuring approximately 7.8 metres north to south and 5.8 metres east to west. The most likely explanation is that this is the remains of an icehouse, a structure common to large demesnes from the eighteenth and nineteenth centuries, where blocks of ice cut from frozen ponds were packed into an insulated underground chamber and used to keep food cold through warmer months. The possibility that it was instead a folly, a decorative garden structure built to create the impression of antiquity, has not been ruled out. Either way, the cartographers of 1911 appear to have taken the mound at its own atmospheric face value.

The townland itself carries a longer history beneath its current name. Knockdrin was formerly known as Monelea, as shown on the Down Survey maps of Rathconnell parish drawn between 1655 and 1659. Those maps, produced in the aftermath of the Cromwellian conquest as part of a vast land redistribution exercise, record that in 1641 the lands of Monelea belonged to Andrew Tuite, listed in the survey as an Irish Papist, the standard designation used to identify Catholic landowners whose holdings were subject to confiscation. The same maps show a medieval castle standing beside a roadway close to the River Gaine in the northern part of the townland, a reminder that the demesne sits on ground that had been settled and contested long before any icehouse or folly was built on its ridge.