Surely you need a valid MEP rating, to be issued with an ME(P)IR?
Your confusion is compounded by sloppy terminology. There is no such thing as an MEIR or, even less, an ME(P)IR.
The privileges of an instrument rating depend upon the class of aeroplane in which you have demonstrated your ability. If you pass the IR skill test on an MEP aeroplane, the privileges are valid on both MEP and SEP. If, on the other hand, the initial skill test is passed in an SEP aeroplane, the privileges are valid only for SEP aeroplanes and a further skill test will be required to extend the privileges to MEP aeroplanes.
Subsequently, IR privileges will depend upon the class of aeroplane in which you passed your last proficiency check. A proficiency check passed in a MEP aeroplane will give IR privileges in both MEP and SEP whereas a proficiency check passed in an SEP aeroplane will give privileges only in SEP.
Clearly, for maximum flexibility, it is preferable to take the initial IR skill test in an MEP to obtain privileges for both classes, in which case the MEP training must be completed. However, if it is anticipated that the privileges will initially be exercised only in SEP aeroplanes, there is no point in taking the MEP skill test and the Aircrew Regulation does not require you to do so. It is unfortunate that so many ATOs do not understand the Regulation in this area (as well as others).