Just passed the Ulster Museum there is a graveyard called Friar's Bush, this site described some of the colourful history that surrounds it.
http://www.friarsbush.org/index.html.
I heard somewhere recently that it was to be opened to the public, I have always wanted to have a poke round, but the gate is always locked.
The guide on the bus tour of Belfast, tells a story about a wall being built underground, to seperate the Catholic and Protestant buried. I cannot find any reference to this anywhere, so I would imagine it is probably nonsense.