O'Brien's Castle is located at one of the island's highest points, in the north of Inisheer.[5]
History
A ringfort once stood on the site, named Dún Formna ("hillfort of the ridge/shoulder", referring to the "shoulder" of land)[6] and dated to between the 5th century BC and 1st century AD.[7][8][9]
The castle is a rectangular donjon (keep). The original doorway was into the first floor, accessed by an external wooden stairs.[citation needed]
The entire first floor was a great hall, with mural stairs up to the parapet. The ground floor was a basement or storage area with three vaulted chambers. The corbel of a bartizan is visible with a human face carved into it.[12]