I think it's slightly more complicated than that. Assuming that you are issued only with a JAA CPL(A), MEIR(A) and MEP(L) Class Rating, you may exercise the privileges of your licence only on a multi-engine aeroplane. You would then need to pass the SEP(L) Class Rating Skill Test to add that rating to your CPL(A). However, you would not need to pass a further IR skill test as a MEIR is valid on a SEP aeroplane provided that you hold a valid SEP class rating