What it is
The castle was built to guard the harbour against pirates and later served as the admiralty court where Cork's mayor claimed jurisdiction over the harbour. Since 2007 it has been run by Munster Technological University as an observatory: the exhibition covers the search for life beyond Earth, there is a working telescope on the tower, and the courtyard café looks onto the river.
The Blackrock to Passage West greenway starts nearby if you want a flat walk along the estuary.
Getting there by bus
Blackrock is about 4 km east of the city centre. Buses along Blackrock Road and through Blackrock village stop within a few minutes' walk of the castle (Ferney Road and Blackrock village stops); the last stretch along Castle Road is on foot.
Allow half a day if you add the greenway or the Marina walk back to the city.
