The rectangular three-storey building, measuring approximately 13 metres north to south and 7.5 metres east to west, showcases the defensive architecture of its era with granite quoins, stepped crenellations, and strategic machicolations at the northeast and southwest corners. Its history stretches back to at least 1616, when Robert French held the castle, likely descended from the nearby Ballytory French family. The property changed hands several times during the tumultuous mid-17th century; Robert Elliott, an Irish Catholic, owned 54 acres here around 1640 before being listed for transplantation in 1653, and by 1666 the Act of Settlement had granted the land to Constantine Neale.

The castle’s defensive features reveal the uncertain times in which it was built. The main entrance on the east wall, protected by an overhead machicolation, once secured with draw-bar sockets and possibly a yett (iron gate), opens directly into the ground floor. This level bristles with fifteen gun-loops strategically positioned in the walls, with multiple loops converging on the corner angles to eliminate blind spots. The upper floors served as living quarters, with the first floor featuring multiple two and three-light windows, plus fireplaces in both the west and north walls. The second floor, supported on corbels, contained additional windows and a fireplace in the east wall, whilst a garderobe chute at the southwest angle provided basic sanitation facilities, though much of the south wall has since collapsed.

Archaeological investigation in 2003, when the castle was conserved as a dwelling, revealed that the structure sits on a layer of introduced clay with a cobbled surface inside. The castle once stood at the northeast corner of a square enclosure measuring roughly 40 metres on each side, still visible as vegetation marks in aerial photographs, though this may simply be the remnant of a 19th-century field boundary shown on the 1839 Ordnance Survey map. Today, Rathshillane Castle offers visitors a tangible connection to the plantation period in Ireland, when fortified houses like this one served both as family homes and defensive strongholds in an era of political and religious upheaval.