Castle Barnagh consists of two distinct rectangular enclosures: a smaller area measuring roughly 21 metres east to west and 20 metres north to south, defined by low earthen banks, and a larger adjoining section on its eastern side spanning approximately 31 metres by 40 metres. These earthworks likely represent the foundations of a medieval tower house and its protective bawn, a fortified courtyard typical of Irish castle architecture.

The site carries considerable historical weight, as it may be the original stronghold of the O’Conor clan, one of the region’s most powerful Gaelic families. This castle, along with its associated church of Cill O Duirthi (now Killaderry), formed an important defensive and religious complex known as Daingean. While some historians have suggested that the later Fort Governor was built directly on top of this earlier O’Conor fortification, evidence points to Castle Barnagh being the original seat of power, with Fort Governor constructed at a new location that would eventually become the nucleus of the sixteenth century settlement of Philipstown.

By 1550, the castle’s glory days had clearly passed; a survey of Offaly from that year refers to the site as ‘Castelberne’ and rather poignantly describes it as ‘an old castell nowe waste’. This documentation provides a fascinating snapshot of a once mighty fortress reduced to ruins, marking the transition from Gaelic to English control of the region. Today, these earthworks offer visitors a tangible connection to Ireland’s complex medieval past, where power shifted between native Irish clans and English colonisers, leaving behind these evocative archaeological traces.