Ballinskelligs Castle is more commonly known as McCarthy Mór Castle. It is in fact a Tower House and was built in the 16th century by McCarthy Mór. It is thought that it's purpose was twofold. Firstly to protect the bay from pirates and also from which to charge a tariff on incoming trade ships.
These Tower houses were built all around the Cork and Kerry coasts by the McCarthy Mór family. Another larger example is Ballycarbery Castle in Cahirsiveen.