This rectangular keep, measuring approximately 14 metres by 9 metres externally, originally rose three storeys from its distinctive battered base. While the western wall and much of the northern and southern walls remain at full height, the eastern section has collapsed entirely, leaving only shallow footings at the southeast corner. The castle once featured a projecting turret at the eastern end of the north wall, though this defensive feature now survives only to the second storey level, with protruding bond stones indicating it originally extended the full height of the structure.

The castle’s defensive arrangements reveal the careful thought that went into its construction. Entry was gained through a doorway in the southern wall, which could be secured by a draw-bar system and likely featured a portcullis operated from the second floor. The ground floor chamber, possibly used for storage or livestock, was illuminated by narrow defensive loops and may have been subdivided by an internal cross wall. A mural stairway built into the thickness of the southern wall provided access between floors, with the first floor serving as the main living space, complete with a wooden floor and pointed barrel vault ceiling. The second floor chamber displays an unusual architectural feature: decorative arcading with segmental arched bays carried on stepped moulded corbels, which both enhanced the room’s appearance and provided additional structural support for the wall-walk above.

Archaeological evidence suggests that Rahinnane Castle wasn’t a standalone structure but rather the centrepiece of a larger complex. Grass-covered foundations immediately south of the tower house outline what appears to have been a substantial rectangular building, measuring roughly 22 metres by 12 metres, which likely served as additional domestic accommodation. This would have housed the everyday amenities such as fireplaces, garderobes and storage spaces that the main tower conspicuously lacks. The castle’s construction technique, using coursed rubble set in sea-sand mortar with wrought sandstone dressings, speaks to local building traditions, whilst surviving traces of wicker centering impressions on vault soffits provide rare evidence of medieval construction methods still visible today.