Originally developed as a coastal defence fortification in the 16th century to protect upper Cork Harbour and port, the site now houses an observatory, visitor centre and restaurant.

[5] In the late 16th century, the citizens of Cork appealed to Queen Elizabeth I to construct a fort at Blackrock to "repel pirates and other invaders".

In 1582 a fortification was built on the site,[6] and later around 1600, a round tower was constructed to safeguard against pirates "carrying away" vessels entering the harbour.

[8] This custom, dating to at least the 18th century,[9] was held every three years in August, and involved a dart (an arrow about four feet long) being thrown by the Mayor from a boat, to indicate the Corporation's jurisdiction over the harbour.

[15] The castle's observatory houses an interactive astronomy center which is open to the public, and has exhibits including a "tour of the universe" and a radio telescope that beams messages composed by school groups towards nearby stars.