Kilquane Cemetery is a well kept small graveyard near the East Cork village of Knockraha. But it has a dark centre. For here there is a vault, known as Sing Sing as it was used as a prison during the War of Independence from 1916 to 1921. It was highlighted this year in a TV3 documentary called In the Name of the Republic. And, sadly, it seems the vault was more of a death row than a prison.
Update 2018: There is quite a lot of info online about this prison. You could start here with this History Ireland article. Or with this Examiner report here.