Mentioned by Dublin 2 days itinerary
The Ultimate 2 Days In Dublin Itinerary
"Dublin Castle is a fine choice and way to begin the first day of your 2 days in Dublin itinerary. Dublin Castle is steeped in history and you will definitely want to take their guided tour, admission fee required. You can opt out and tour the grounds by yourself, but you will not have access to the staterooms on the second floor. I took the tour and found it fascinating."