His construction project would prove to be his undoing; the Annals of the Four Masters records that upon completion of the castle, de Lacy went to inspect his new fortification accompanied by three Englishmen. A local youth named Gilla-gan-inathar O’Meyey, whose name rather unkindly translates as ‘gutless lad’, approached the lord with a concealed axe and delivered a single fatal blow, severing de Lacy’s head from his body. Both parts tumbled into the castle ditch, the killing carried out ‘in revenge of Columbkille’, suggesting the murder was retribution for perceived sacrilege against the nearby monastery.

The motte stands today as National Monument No. 313, a substantial mound rising eight metres high with a base diameter of 43 metres, situated within the landscaped grounds of Durrow Abbey House. Aerial photographs from 1973 and historical sketches suggest the Anglo-Norman fortification was deliberately built within or on the line of the earlier monastic enclosure, a pattern seen at other Irish ecclesiastical sites like Seir Kieran. The strategic placement on slightly elevated ground gave the castle command over the surrounding area, whilst its proximity to Durrow’s church, graveyard and high cross, just 180 metres to the east-southeast, hints at the complex relationship between Norman military power and existing Irish religious sites.

During the 19th century, the residents of Durrow Abbey House transformed this medieval defensive structure into a romantic landscape feature. They added a wall around the summit with an arched eastern entrance and erected what the 1908 Ordnance Survey map marks as a summer house or gazebo, though this structure now exists only as a depression nearly a metre deep atop the heavily overgrown mound. Pathways were laid encircling the base and a walled garden created immediately to the west, effectively converting what had been the site of Hugh de Lacy’s violent death into a genteel Victorian garden folly.