The site consists of two distinct rectangular areas: a smaller enclosure measuring roughly 21 metres east to west and 20 metres north to south, which likely marks the footprint of the tower house itself, and a larger adjoining area on the eastern side spanning approximately 31 metres by 40 metres that probably served as the bawn, or fortified courtyard.

This unassuming archaeological site may hold particular significance in the history of the O’Conor clan, who dominated this region for centuries. Some historians believe Castle Barnagh could be the original fortress of Daingean, an important O’Conor stronghold that controlled the surrounding territory. The castle was strategically linked with the nearby church of Cill O Duirthi, now known as Killaderry, forming a classic pairing of secular and religious power that characterised medieval Irish lordships.

The relationship between Castle Barnagh and the later Fort Governor adds another layer to the site’s complex history. While tradition long held that Fort Governor was built directly atop an earlier O’Conor fortification, archaeological evidence suggests a different narrative; Castle Barnagh may have served as the original O’Conor castle, which was eventually abandoned in favour of Fort Governor at a new location. This relocation would have been strategic, as the sixteenth century settlement of Philipstown subsequently developed around Fort Governor, marking a shift in both the political and urban landscape of the area.