Rather than the typical square or rectangular stronghold, this fortress consists of a massive circular wall, roughly 45 metres in diameter and originally topped with three defensive towers. The circular enclosure, built from large blocks of local grit stone, may date back to the 12th century, possibly erected by the O’Donovans, the Irish chieftains who controlled these fertile plains before the Anglo-Norman invasion. Its design bears a striking resemblance to Caislean na Caille in Lough Mask, suggesting it was built to defend against the encroaching Norman lords who coveted the rich lands of Hy-Figeinte.

The site carries a dark history; tradition holds that in 976, Donovan lured King Mahon to a conference here before betraying him to his death, an act later avenged by Mahon’s brother, the legendary Brian Boru. After the O’Donovans were expelled around 1178, the de Lacy family took possession and likely added the three tower houses that once crowned the circular wall. By the 1840s, only two of these towers remained intact, with the eastern tower standing an impressive 18 metres high, accessed through a pointed doorway of considerable strength. This tower featured at least five storeys connected by stone stairs, whilst the northwestern tower, though lower at about 12 metres, was broader and more massive in construction, complete with a dungeon that once bore iron fastenings for chains.

The fortress passed through various hands over the centuries; the de Mareys family, the de Lees, and eventually back to the Lacys, who held it until Cromwell’s forces dispossessed them. The 1654 Civil Survey recorded it as comprising “three small unrepaired Castles, & a bawne,” along with a grist mill and eel weir. Today, much of the circular wall survives to a height of about 7 metres on the river side, though portions have been pilfered over time to build farm structures within the enclosure. Despite this degradation, the remaining ruins offer a remarkable glimpse into a defensive structure that bridges the gap between native Irish fortification traditions and Anglo-Norman military architecture.