Located on a small island in Lough Derg, the castle’s exact construction date remains uncertain, though historical records suggest it was built sometime in the medieval period when the O’Briens controlled much of Thomond, the ancient kingdom covering modern Clare, Limerick, and parts of Tipperary.

The castle would have served as both a defensive stronghold and a symbol of O’Brien authority over the strategic waterways of Lough Derg. Its island location offered natural protection; boats were essential for any assault, giving defenders a significant advantage. The O’Briens were known for their network of castles and tower houses throughout Clare, using them to maintain control over their territories and collect tributes from local clans and merchants travelling the Shannon waterway.

Today, only fragments of the castle walls remain visible among the island’s vegetation, offering glimpses of what was once a formidable structure. The site represents part of Ireland’s complex medieval history, when Gaelic lords like the O’Briens maintained their own laws and customs whilst negotiating, and occasionally fighting, with Anglo-Norman settlers and the expanding English crown. Though the castle is now largely forgotten compared to more famous O’Brien strongholds like Bunratty, these ruins on Inishmacowney continue to tell the story of Gaelic Ireland’s political landscape and the strategic importance of controlling Ireland’s inland waterways.