Built between 1200 and 1220, this five-sided fortress features a massive circular keep at its northern angle, with walls five metres thick and an internal diameter of just over seven metres. The castle’s distinctive design includes flanking towers at the eastern and western corners, a formidable twin-towered gatehouse to the south, and a substantial curtain wall that once enclosed the entire complex. Archaeological evidence, including a coin minted between 1205 and 1218 found in the gatehouse foundation, confirms its early thirteenth-century origins.

The keep itself rises four storeys high, topped with a Victorian addition from the nineteenth century. Its second floor served as the main public hall, complete with a fireplace featuring a sloping stone hood similar to that found at Roscrea Castle, whilst the third storey housed the lord’s private residential quarters. These upper chambers showcase remarkable architectural details including chevron-decorated arches, sandstone windows, and the distinctive shouldered or Caernarvon arches that likely date from late thirteenth-century renovations. The gatehouse complex, recent excavations reveal, was constructed in two phases; a sophisticated two-storey hall with rib vaulting supported by central pillars was added to the rear of the twin-towered gatehouse in the latter half of the thirteenth century.

The castle’s turbulent history reflects Ireland’s political upheavals across the centuries. It passed to the Mac Ibrien family in the fifteenth century before returning to Butler ownership in 1533, survived an attack by prisoners who burned its gates in 1332, and was later repaired by Colonel Abbott during the Commonwealth period. Following the Williamite wars, it was deliberately dismantled as a precaution against future rebellions, and in the eighteenth century, Solomon Newsome attempted to blow it up entirely, leaving the large breach still visible in the north wall at ground level. Despite these assaults, Nenagh Castle remains a remarkable testament to Anglo-Norman military architecture and the complex political landscape of medieval Ireland.