City bound people will never have been as far as Balbriggan, but I assure you it is in Dublin. Ardgillan Castle was once the home of the O'Caseys, later owned by the Earls of Tyrconnell and then by the Taylor family. It has belonged to Dublin and later Fingal County Council for the last 20 years. The current structure is only really a castle in name, like Malahide Castle, no one has defended it from the battlements.
I took the below photo today just before a massive thunder storm. The rain was so bad I had to pull in on the motor way and wait for it to subside. To get to Ardgillan, head up the M1 and take junction 5, then follow the signs for it through Balrothery. It is accessible by train and bus, but I wouldn't recommend the bus journey; it goes through Swords, Rush, Lusk and Skerries before it gets to Balbriggan, but it does stop right outside the end of the demesne.