In 1602, Donal Cam O'Sullivan Beare was clan chieftain. He had rebelled against the English crown and was a fugitive after the fall of Dunboy Castle. According to legend, his wife was murdered by an English military commander; St. Leger and Donal swore revenge. He went to Carriganass Castle, then occupied by St. Leger, disguised as a monk and persuaded him to confess to the murder. Donal then killed St. Leger by throwing him from the top of the castle tower into the river below. Donal then jumped from a window and escaped.
The castle served as a military stronghold for less than 100 years. Later, it was used as a farm and acquired by the Barrett family. In 1880, cornerstones from the tower were used to renovate Bantry House. The O'Sullivans repurchased the castle from the Barretts in the 1930s. In 2002, the O'Sullivan family donated the castle to the community.