This rectangular structure, measuring just over 7 metres east to west and 5.5 metres north to south internally, dates likely to the 17th century. The Down Survey map of 1655-6 marks it as a ‘decayed castle’, suggesting it was already in ruins by the mid-1600s, though records show it was owned by Thomas Prendergast in 1640. Today, its walls survive to heights between 1.5 and 3 metres, except for the eastern wall which has completely collapsed.

The castle’s construction tells an interesting story of both original design and later modifications. The walls are notably thin, ranging from 35 to 50 centimetres in width, and a single window opening pierces the western wall. A semicircular bulge on the southern wall’s eastern end once housed a newel staircase, providing access to upper floors that have long since vanished. Curiously, there’s no evidence of either an entrance or vaulting, which raises questions about the building’s original form and function.

Later inhabitants clearly adapted the structure for their own purposes, adding internal wall foundations and installing an oven in the southwest corner. These secondary features suggest the building continued to serve some domestic or agricultural purpose even after its days as a defensive structure had ended. The absence of typical castle features like thick walls or defensive elements, combined with its 17th-century date, places Kilgainy Castle within that transitional period of Irish architecture when fortified homes were giving way to more comfortable, if less secure, residences.