The site takes its name from Cill Bhiscí, meaning Bigseach’s church, indicating it was once home to an early religious foundation. Following the Norman invasion, Hugh de Lacy granted these lands to Geoffrey de Costentin, who transformed the strategic location into a formidable motte and bailey castle in 1192, as recorded in the Annals of Lough Cé.

The impressive earthwork rises 11 metres high, its conical mound measuring 32.5 metres across at the base and tapering to an oval summit of 19 by 17 metres. Archaeological surveys have revealed fascinating structural remains on top, including foundations of two rectangular buildings; a smaller eastern structure measuring 2.7 by 2.5 metres built on a plinth, and a larger western building of 6.6 by 2.8 metres. The motte, constructed from earth and boulders quarried from deep hollows on the north and south-east sides, was originally encircled by a six-metre-wide defensive ditch with a substantial counterscarp bank. A peculiar lunate inner bailey between the fosse and the western bank hints that the Normans may have built upon an earlier Irish rath, adapting existing earthworks for their purposes.

Medieval documents from Tristernagh priory, dating between 1200 and 1224, paint a vivid picture of a thriving borough established here by de Costentin, complete with burgages, a church, mill, hospital known as the Leper House, and even a reference to the house of an old Irish chaplain. The castle remained the centre of the de Costentin manor until the family line ended in the mid-fifteenth century, after which it gradually fell into obscurity. Today, visitors can still trace the outlines of the deserted medieval settlement in the surrounding fields, marked on old Ordnance Survey maps as ‘Kilbixy Town (site of)’, whilst the commanding views from the motte summit remain as impressive as they were eight centuries ago.