Built in the late 15th or early 16th century, this square fortress originally rose several storeys high within an extraordinary archaeological complex that dates back to the 7th century. The castle shared this sacred ground with St. Manchán’s monastery, which included a multi-period church featuring Romanesque and late medieval elements, early Christian cross-slabs, and various religious structures including a holy well, a possible hermitage, and an ancient causeway called a togher that connected the island to the mainland.

The castle itself was a sophisticated structure for its time, featuring a lobby entrance design with main chambers on each floor connected by spiral stairs. Its windows were single light openings with elegant ogee heads and stepped recessed spandrels, whilst an ingenious double garderobe system at ground level served toilet shafts on the upper floors; a similar arrangement can be seen at nearby Togher Castle. Mural passages at the second floor likely led to additional garderobes positioned in the castle’s corners, lit by narrow defensive loops. Among the castle’s more intriguing features was a sheela-na-gig, a medieval stone carving of a female figure, which was documented by Thomas Cooke of Birr in 1870 but has since vanished along with his drawing.

Today, only the ivy-covered southwest corner remains standing after the main structure was demolished in 1959, reaching about 2.5 metres in height with walls nearly a metre thick showing evidence of a defensive base batter. Scattered on the ground nearby lies half of a two-centred doorway with punch dressing, confirming the building’s late medieval date. As one of 28 castles in Mac Coghlan’s Country, Lemanaghan played a significant role in local history; it was here in 1627 that Conell Mageoghegan translated the Annals of Clonmacnoise into English under the patronage of Terence Coghlan of Kilcolgan Castle. The estate had been granted to Oliver St. John, Lord Deputy of Ireland, in 1621 as part of the Plantation of Delvin Mac Coghlan, marking the transition from Gaelic to English control of this ancient site.