This National Monument (No. 404) showcases the architectural evolution of a single site from medieval fortress to Georgian mansion, with each era's builders incorporating rather than demolishing what came before.

The castle's origins date back to around 1317, when the de Wogan family received a land grant and likely built their stronghold shortly afterwards. The original 14th century structure, now visible in the northeast corner of the complex, was a robust rectangular building measuring approximately 15.5 metres east to west. Its thick rubble masonry walls, nearly two metres wide, supported a barrel-vaulted ceiling over three metres high, with defensive arrow loops positioned strategically in the south wall. The castle weathered an attack in 1454 and was later fortified again in 1641, adapting to the changing military needs of successive centuries.

What makes Rathcoffey particularly fascinating is how subsequent owners chose to build around and incorporate the medieval castle rather than tear it down. The 17th century saw the addition of a fortified house attached to the western side of the original structure, complete with brick-vaulted ground floors and back-to-back fireplaces. Then, sometime after 1785, an elegant three-storey Georgian mansion was constructed, its south-facing facade featuring a refined arcade at ground level. Today, though render and plaster obscure many architectural details, visitors can still trace the building's evolution through blocked doorways, inserted windows, and the remnants of wooden staircases that once connected these different historical phases. The site commands panoramic views from north to west-southwest, and keen-eyed observers might spot low earthworks to the north and east; remnants of the late Georgian gardens that once graced the estate.