The present castle was designed by the Pain brothers, it was built between 1828 and 1829. It was used by Cork Corporation for civic functions during the nineteenth century. The Mayor of Cork had been Admiral of the Port since 1627 and Admiralty Courts were held in the castle also. Every third year, on the first of August, the Mayor and other members of the Corporation set out on boats from Blackrock Castle for the ceremony on 'throwing the dart'. When the boats had reached the mouth of Cork Harbour,the Mayor threw an arrow about four feet long into the sea. The ceremony symbolised the authority of the Mayor and Cork Corporation over the harbour and has been revived in recent years.

The castle was in private hands for many years but it is once again owned by Cork City Council. It has been restored and houses an observatory, operated by staff from the Cork Institute of Technology, and a restaurant.