This early seventeenth-century fortified house, bearing a datestone of 1601, represents a fine example of Z-plan architecture; a defensive design that became popular in Ireland during this turbulent period. Built from roughly coursed limestone rubble and rising four storeys high, the castle consists of two diagonally opposed rectangular towers that create the distinctive Z-shaped footprint. The larger southeast tower housed the main living quarters, whilst the smaller northwest tower contains a remarkable stone staircase with an unusual stone banister that remains intact to this day.

The castle's defensive features reveal the uncertain times in which it was built. The main entrance on the southwest wall consists of a round-arched doorway with rebated jambs, protected by both an external machicolation at wall-walk level and an internal murder-hole; a sobering reminder of the violence that could erupt at any moment. Windows throughout the building are narrow, flat-headed single and twin lights with hood-mouldings, designed to admit light whilst minimising vulnerability to attack. Particularly interesting is a crossloop at first-floor level on the southwest wall, with a large circular gun port positioned directly above it. At roof level, five tall rectangular chimney stacks with distinctive funnel-shaped caps punctuate the skyline, whilst bartizans at the north and south angles provided additional defensive positions for the castle's occupants.

According to historical records, Killaleigh was an O'Connor house, though the building underwent significant modifications during the nineteenth century. These later alterations included the insertion of barrel-vaulted roofs between ground and first-floor levels, the replacement of the original stone stairs with wooden ones, and the addition of two flat-headed doorways in the northeast wall. Unfortunately, no original fireplaces or interior architectural features remain visible today. A possibly original bawn wall stands to the southwest of the fortified house, though it too has been altered with nineteenth-century outhouses attached to its internal face. Despite these changes, Killaleigh Castle remains an impressive testament to the defensive architecture of early modern Ireland, when powerful families like the O'Connors needed both comfortable homes and formidable fortresses.