I would coat the walls with stucco. You can find numerous finishes and colors. It is basically like plaster work and should adhere well to brick. Clean off all loose debris and pre-moisten the brick. The stucco may be applied smooth or textured. Can also be coated with paints after allowing time for full cure. You could plaster, but I think stucco, as a portland cement product, will hold up to occasional moisture much better.
So brush up on your masonry skills and give it a go.