The CPL does have an instrument content, but it is not applied instrument flying, therefore you do not need an IRI to treach the JAA CPL on single engine aircraft. The requirements to teach the CPL are 200 hours of instructional time and be an unrestricted instructor. No further rating is required other than an approval from the school you are working for.
To teach IR or IMC, if you hold an instructors rating, then to remove the applied instrument flying restriction you will need to do the IRI upgrade course, (there is a stand alone IRI for those not holding an FI rating but it requires alot more actual instrument flying experience). The number of instrument hours required to do the course is 200 hours IFR, but there is some 1 to 4 factoring ratio such that 1 hour in real IMC or flight to sole reference to instruments can be counted as 4 hours under IFR. I can't remember the exact terms so look in lasors, I'm sure someone can correct me.
To do the MEI you will need 30 hours of p1 MEP time prior to doing the MEI upgrade course. Once you have both the MEI and IRI you can teach multi IR.