Where do you want to teach? You can only teach for the IR at an approved FTO. If the FTO teaches Integrated courses an IRI rating cannot be used as the students are regarded as ab-initio. This limits the IRI to teaching on modular courses. The majority of these will be for the ME IR therefore in addition to completing the IRI Course you will also need to complete the CRI(ME) Course JAR-FCL 1.390(b) which requires a minimum of 30 hours PIC on the Class or Type.
Once qualified as a IRI or CRI(ME) you will get a 25 hour credit against the Teaching and Learning element of an FI Course (App1 to JAR-FCL 1.395 and App 2 to JAR-FCL 1.380) That still leaves 100 hours of theoretical training plus 30 hours of Flight training to do to get the FI rating.
In 11 years of JAR-FCL I have only ever come accross one standalone IRI and that was whilst he converted his military ratings to a FI.