How can I add tile to my fireplace?
Cement surface and even paint (scarifed with 50 grit belt sander or similar) can stay, if both are well adhered..  

An experiment is in order:  


Use a good quality thinset, (Laticrete Platinum 254 or Custom Flexbond)
Trowel on 1/4 inch patch (4in x 4in or so).
Wait 24 hours
Chip off patch with a chisel


Observe the removed thinset:  


If it cleaves off with no embedded paint, paint is very well adhered, but may be too slick, scarify through paint layer to cement before tiling.
If some paint embeds, and some thinset remains on surface, it is a good interface layer, proceed with tiling.
If paint cleaves off easily, remove paint fairly thoroughly before tiling.


In no case use a premix mastic thinset or other adhesive to adhere tile.