Built in 1656, the ruins of the star-shaped Cromwell Fort are situated on the most eastern side of Port Island.
It is told that the O’Malley family built a castle on Dun Grainne in the 16th century and Don Bosco, an ally of Grainne Uaile (female chieftain of the O’Malley family) built a castle opposite Dun Grainne where the ruins of Cromwell’s Barracks now stand. Together the two castles controlled the surrounding waters.
In the later years, during the Cromwellian period, a star shaped fort was built as use of barracks to house captured Catholic clergy found guilty of high treason. From here, they waited to be shipped to the West Indies and other remote places.
After Charles II was restored to the throne in 1660, the fort was used as part of a defensive strategy and later used by Irish forces during the Jacobite War.
The fort now lies in ruins but there are many architectural features still be seen.