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 or around 1600, a round tower was constructed to safeguard against pirates "carrying away" vessels entering the harbour. Earliest remains today are a circular tower right on the water's edge, 10.5m in diameter, with walls 2.2m thick to withstand cannons.
Blackrock Castle was in the ownership of the City of Cork following a charter of James I to the City in 1608. The castle is later referred to in the Council Book of Cork in 1613 and 1614. In 1722 the old four-storey tower was destroyed by fire and new one built by the citizens, costing £296.
Throughout this period, the castle was used by the Corporation for banquets and 'convivial gatherings', some associated with the custom of 'throwing the dart'. This ancient custom was held every three years in August, where a dart (an arrow about four feet long) was thrown from a boat by the Mayor accompanied by officers of the Corporation.
Following a banquet, the castle was destroyed by fire in 1827. The rebuilding began at the direction of Mayor Thomas Dunscombe in 1828 and was completed in March 1829. The architects added three additional storeys to the original tower and rebuilt the out-buildings. The new building cost the City of Cork £1,000. The architects were James and GR Pain, who were responsible for many fine churches and public buildings in Cork, notably the Courthouse in 1835, the interiors of the Catholic Cathedral of St Mary and St Anne in 1828, and St Patrick's Church in 1836. This neo-gothic complex of buildings around a courtyard is essentially what remains today.
The building was purchased by Cork Corporation in 2001 and had previously been used at different times as offices, as a restaurant and as a private residence.
Blackrock Castle houses Ireland's first fully interactive astronomy center developed by the award-winning multimedia company Martello Media. The exhibition is now open to the public and is themed "The Search for Extreme Life in the Universe". Highlights of the exhibition will include:
- A tour of the Universe using interactive floor-to-ceiling screens. Visitors will be able learn about the evolution of the universe, from the earliest moments to the present day and beyond, delving deeper into the interactive software if they wish, or taking a more leisurely tour where they can enjoy the experience as a spectator.
- Ireland's first "social software" cinema designed by Martello Media, with the digital post production by the award-winning Irish company The Farm, especially for visitors to Blackrock Castle. In our cinematic experience the audience directly influences the design of a space mission that may be needed to divert a comet from colliding with the Earth, using scientific principles to make critical decisions along the way.
- A radio telescope that will actually beam messages composed by school groups towards nearby stars, some with known planets in orbit around them. A simulator will allow users to follow their signal as it travels in real-time out of the solar system and on towards its selected destination.
- An optical telescope that will continuously search for very short flashes of light (lasting mere billionths of a second) that may be evidence for extraterrestrial intelligence. This technique, known as an "Optical Search for Extraterrestrial Intelligence" (OSETI), will use state-of-the-art instruments built by companies in Cork, including SensL, a photonics company.
The castle already houses a team of astronomical researchers from Cork Institute of Technology who have been working on the development of the exhibition and the construction of Ireland's first robotic observatory. At the same time, the team are currently engaged in a number of astronomy projects.