The castle mound at Threecastles Demesne rises nine metres high with a flat top spanning roughly 55 metres across, surrounded by a defensive ditch on its southern side. The site takes its medieval name 'Odagh' from the Ui Duach tribe who held these lands before the Anglo-Normans arrived; following the Norman conquest, the area became part of the Manor of Odogh within Earl Strongbow's Lordship of Leinster, granted by Henry II.

This substantial earthwork represents one of the earliest Norman fortifications in the area, though its original features have been significantly altered over the centuries. According to the historian Carrigan, writing in 1905, the motte's summit was extensively remodelled during the late 19th century, obliterating much of its ancient character. Today, visitors will find the top covered with a curious network of pathways, about two metres wide, winding between raised earth banks and platforms that reach heights of up to 2.5 metres. A small castellated folly, built during the Victorian era, sits on the northern edge, whilst the motte's sides have been landscaped into a series of steep slopes alternating with narrow terraces.

Despite these later modifications, the motte remains an imposing presence in the landscape, offering commanding views across the Nore valley in all directions. A lime kiln, built into the outer face of the southern ditch, provides evidence of the site's continued use for industrial purposes long after its military significance had waned. The northern side lacks the defensive ditch found elsewhere, instead sloping gradually down towards the river, taking advantage of the natural topography for defence.