Built by the O’Brien clan during the 15th century, the castle served as an important defensive position overlooking the Shannon Estuary. The structure took its name from Donnell O’Brien, who established it as part of a network of fortifications that helped control this strategic waterway connecting Limerick to the Atlantic Ocean.

Throughout its active years, the castle witnessed considerable turbulence, changing hands multiple times during the various conflicts that swept through Munster. The English seized it during the Elizabethan conquest of Ireland, and it later played a role in the Confederate Wars of the 1640s. By the time of the Cromwellian invasion in 1651, the castle had already suffered significant damage, and Cromwell’s forces completed its destruction to prevent it being used as a rebel stronghold.

Today, only scattered stone foundations and earthworks remain visible at the site, which sits on private farmland near the village of Cratloe. The ruins, though modest, offer a tangible connection to the complex medieval history of County Clare; a time when control of the Shannon meant control of trade, movement, and political power in the region. Local historians have identified several architectural features including the remains of a bawn wall and what appears to have been a substantial tower house, typical of Irish castle construction during that period.