Historical records place the castle on the Down Survey map of 1655-6, a comprehensive mapping project undertaken during the Cromwellian conquest of Ireland. By the time the Ordnance Survey mapped the area in 1837, the structure had already vanished, leaving only a notation marking the ‘Site of Castle’ on their detailed six-inch maps.

Today, visitors to the site will find an approximately square plot measuring roughly 30 metres on each side, now planted with trees. The only visible evidence of the castle’s existence lies in a modest foundation course of boulders, stretching at least four metres in length, which can be spotted exposed in a field drain at the southeast corner of the site. These weathered stones serve as the sole physical reminder of what was likely a tower house or small fortified dwelling, typical of the defensive structures built throughout Ireland during the medieval period.

The site’s transformation from active fortification to tree-covered plot reflects the broader changes in the Irish landscape over the centuries. Like many similar structures across County Carlow, Killerrig Castle likely fell into disuse during the upheavals of the 17th century, its stones gradually repurposed for local building projects until only the deepest foundations remained undisturbed in the soil.