I think the requirement to have a valid MEP IR is for the initial multicrew type rating. Once you have a multi crew type rating on your license, things become easier.
In any case your license should include an IR specific to the aircraft you have just finished flying, which I presumbe will be a multi engined aircraft of some sort?
Section F4 in Lasors gives more info.
Pre-requisite conditions for training
An applicant for the first type rating course for a MPA shall
provide evidence that the following requirements have
been met:-
a. have completed at least
70 hours as pilot-incommand
of aeroplanes;
b. hold a current and valid multi-engine Instrument
Rating (Aeroplanes). This is only a requirement
for an initial MPA type rating and not subsequent
type ratings.