Try a 1/4 or 3/8 roller cover. If this isn't smooth enough then you will have to use a sponge roller.
Make sure to prime the new plaster walls really well. This coat, the foundation for the paint, often can be lightly sanded to a very smooth surface. This will help. Also, you can try thinning your emulsion (paint) a little to help it flow out better on the surface.