This O’Reilly stronghold, built in the 16th century, commanded the river crossing from atop a modest mound standing 1.8 metres high. The castle keep formed the heart of a defensive complex, surrounded by a roughly rectangular bawn measuring 53 by 32 metres, with projecting towers at the southeast and southwest corners providing additional protection on the vulnerable landward approach.

The castle’s builders took full advantage of the natural rock to create formidable defences. They carved deep fosses, or defensive ditches, into the stone itself; the eastern and western ditches stretched approximately 4.2 and 5.1 metres wide respectively, whilst a narrower, shallower fosse protected the southern side where the main entrance likely stood. The bawn walls rose about 1.6 metres above these fosses and the river level, creating multiple layers of defence that any attacker would need to overcome. After falling into disuse, the O’Reillys reoccupied the castle in the 17th century, though local tradition holds that Oliver Cromwell’s forces ultimately sealed its fate during his Irish campaign.

Today, nothing remains visible above ground at Bellanacargy, though the site’s strategic importance is still apparent from its commanding position over the river. Archaeological surveys conducted in 1942 and subsequent research by Davies in 1947 documented these now-vanished fortifications, preserving knowledge of this once-formidable O’Reilly stronghold that controlled this crucial river crossing for over a century.