Pre-requisite conditions for training for a multi-pilot type rating are given at JAR-FCL 1.250. Paragraph (a)(2) states "have a valid multi-engine instrument rating (A)". JAR-FCL 1.005(a)(2) states "Whenever licences, ratings, authorisations, approvals or certificates are mentioned in JAR–FCL, these are meant to be licences, ratings, authorisations, approvals or certificates issued in accordance with JAR–FCL."
Therefore, you must hold a multi-engine IR(A) issued in accordance with JAR-FCL before commencing training for a multi-pilot type rating. An ICAO IR will not do and you cannot do the JAA IR at the same time as the MPA type rating.