The castle’s story begins in the medieval period when these lands formed part of William Marshall’s grant to Tintern Abbey, confirmed by Richard II in 1382. By the early 15th century, the castle had passed into the hands of John Barry, later transferring to John Allen in the 17th century. The Civil Survey of 1654-6 records that the ruined castle and its 240 acres belonged to Sir Caesar Colclough of Tintern, who had acquired the former abbey’s extensive holdings after the dissolution of the monasteries in 1541.

The castle itself is an oblong structure measuring 25 metres north to south and 9 metres east to west, built with a practical layout typical of its era. At its heart lies a central hall, with a kitchen positioned to the north accessed through an east-west passage, whilst a private chamber with fireplace occupies the southern end. Though time has taken its toll on the internal walls, several architectural features remain visible: three double-splay windows, a slop stone, and a doorway leading from the kitchen to what was once a bawn on the western side. The first floor housed additional chambers with fireplaces above the ground floor’s southern chamber, and an offset tower to the east and south originally contained stairs and a garderobe.

The castle sits at the northeast corner of what was once a bawn, an enclosed courtyard measuring approximately 38 by 31 metres, though only slight scarps and tie-stones in the castle’s western wall now hint at its former boundaries. About 350 metres to the east-southeast lies the site of Rathumney church, connecting the castle to the area’s broader religious landscape. Now designated as National Monument No. 229 and in state ownership, Rathumney Castle continues to intrigue visitors and historians alike, offering glimpses into the lives of the Anglo-Norman and later Irish families who called it home through the turbulent centuries of Irish history.