Ardgillan Castle is about 20 miles north of Dublin near Skerries. You can stroll around the gardens and pay to view the castle. Dont have a photo of it, but the next time I'm in Skerries, I will take a pic for VT posterity ;)
Just 20 km north from Dublin airport and you find yourself in beautiful place. With its lovely harbour and long, sandy beaches, Skerries is a great little town, good place for holidaymakers.