Kilmainham Gaol was "built in 1796" and is a "witness to Ireland's turbulent passage from the late eighteenth to early twentieth century. Prisoners from the United Irish Rebellion of 1796, the Emmet Rebellion of 1803, the Great Famine of 1845 to 1851, the Young Ireland Rebellion of 1848, the Fenian Rebellion of 1867, the Land War of the 1880s, the Easter Rising of 1916, the War of Independence and the Irish Civil War were held and often executed here."
They actually imprisoned children here as well as adults. It was quite interesting, especially in light of the fact that we had just recently visited Alcatraz. Wikipedia discusses this and provides a list of the former prisoners.