Local tradition holds that the castle was built by Beithne O’Brien, the same figure who constructed and lent his name to the nearby Church of Kilbeahany. By 1840, when the Ordnance Survey documented the site, much of the structure had already fallen into ruin, with the south and west walls completely destroyed save for a small fragment. What remained revealed an impressive five-storey tower house, measuring 24 feet in length and 18 feet in breadth internally, with walls reaching about 60 feet high and an impressive 4 feet 6 inches thick; testament to the defensive needs of medieval Ireland.

The castle’s documented history begins in 1502 with the death of Donchad O’Brien, lord of Pubblebrian and Aherloe. By 1584, ownership had passed to Mahowne mac Murrogho O’Brien, though his participation in rebellion would see the castle attainted and transferred to Edmund mac Gibbon, known as the White Knight. The White Knight made his mark on history here in 1601 when he captured the Sugan Earl and brought him to Kilbeheny as prisoner. Following the White Knight’s death in 1608, the castle passed through his family until 1611, when it came into the possession of his granddaughter, Lady Margaret Fenton. The castle’s strategic importance continued into the Cromwellian period; Oliver Cromwell himself wrote to Lenthall in 1650 that he had taken possession of “a Castle called Kilbenny, upon the edge of the County of Limerick”, where he stationed 30 foot soldiers.

By 1938, visitors found the castle in an advanced state of decay, with one entire side collapsed to expose the internal vault. Despite this deterioration, architectural features remained visible, including passages within the walls where stairs once stood, a small square window in the basement room, and a corner window high in one angle of the building. The vault itself could still be climbed, offering brave visitors a glimpse of the castle’s former grandeur. Today, these atmospheric ruins continue to draw those interested in Ireland’s turbulent medieval past, standing as a physical link to the O’Briens, the White Knights, and even Cromwell’s military campaigns through Munster.