rendering on to mixed shaped stone wall

[fishface400]

I am looking to render part of a gable end wall. The wall is made of mixed shaped stones. In built into the wall in brick is a chimney. The shape of the chimney is rendered on the exterior to hide the brickwork but needs chipping off and re-rendering.
I'm looking for a mesh type product to screw / secure to the surface of the wall to help prevent cracking and help application or should i just render on to the surface?
Any advice gratefully received