Carrigrenan Tower, lies on Little island in Cork Harbour, in County Cork in Ireland.
I was not able to find any historical information about this tower. Although it is often called a castle on the internet, I don't think it is.
Carrigrenan Tower is situated on the former grounds of Carrigrenan House, on the shore of Lough Mahon. This house stood on the site of the waste water plant that now occupies most this southern peninsula of Little Island. In the planning documents of this plant it is called an 'ornamental tower'. So, I guess this was a folly or lookout tower of the owners of Carrigrenan House. If that is the case, then the tower would probably have been built somewhere in the 19th century.
The ruin of Carrigrenan Tower is freely accessible. A nice tower on a very nice location.