One of my favorite places I’ve seen in Dublin so far is Kilmainham Gaol. A historically significant jail that held prisoners from the Irish Rebellion and Civil War, the Gaol is a visually interesting building constructed in the Panopticon style.
Originally built in response to poor conditions in other prisons, it was subject to similar overcrowding issues with its use as transport hold for prisoner’s being shipped to Australia, the Great Famine, and the criminalization of the mentally ill.
There is a half hour and one hour tour. Both are very reasonably priced at €4 and €9. I would recommend the hour tour plus there is a small museum at the end with artifacts and more info from the Rebellion and Civil War.