The castle, likely a tower house, once commanded this peninsula with defensive walls cutting across the neck of the promontory, as shown in late 16th century maps. These same maps reveal that by then, the bastioned Duncannon Fort had already been built around the earlier medieval structure, creating a layered military complex that would defend this crucial waterway for centuries.

The castle’s ownership changed hands frequently during the turbulent Tudor period. John Inglysshe held Duncannon in 1541, followed by Sir Osborne Itchingham in 1545 and Sir Nicholas White, the Master of the Rolls, in 1569. The site gained notoriety in 1580 when John Itchingham, suspected of conducting illicit trade with pirates, barricaded himself inside and resisted arrest by the sheriff. This episode highlights how these coastal fortifications often served dual purposes; official military outposts by day, potential smuggling dens by night.

Until the early 20th century, visitors could still see a circular tower from the original castle, known locally as King James’ Tower. This last tangible link to the medieval fortress survived within the parade ground of the later fort, but has since vanished, leaving only historical records and archaeological traces to mark where this once formidable castle controlled access to Ireland’s southern rivers. Today, while the fort remains, the castle exists mainly in historical documents and the occasional stone foundation glimpsed beneath the parade ground’s surface.