Castlebawn is a 16th-century Tower house, in County Clare, Ireland. It is on a small island on Lough Derg on the River Shannon, close to the shoreline in Scariff Bay. It was built by the McNamaras about 1540, blown-up in 1827, and now restored and opened to the public.
Castlebawn was built by the McNamara chieftains during the mid to late 16th century. Records show that Castlebawn was built by Owen McNamara, but do not give the year of construction. The earliest recorded mention of Castlebawn is 1570, when Sean McNamara, Chief of eastern Clann-Cuilein, died, leaving Castlebawn to his son, Sir John McNamara. McNamaras lived there until the Cromwellian land confiscations of the 17th century when it became disused. In 1827 the towerhouse was badly damaged by explosives when the authorities attempted to destroy it, as it had become a den for local Poitín makers. The castle was so well built that three of the walls remained intact.  Simon Flannery occupied the castle early in the 20th century, and locally the castle is still referred to as Simon's Castle.
In 1996 Pat and Mary Cody completed the restoration of the ruin, which is now open to the public in the summer months, although access is via boat only.