Is there a "single instrument rating"?
Good point SASLess.
I have never seen a JAA licence with a "Single IR" rating or twin for that matter.
There is only "IR" and then the pilot may have type ratings on twins or singles in his license. JAR-FCL does not prohibit single IR flying in any way.
JAR-OPS hovewer does not allow single IFR flights for transport operations but that has nothing to do with flight crew licensing.