I’m planning to do an IRI/ME rating over the winter.
I think you mean that you want to do an IRI Course which you can do with 800 hours IFR time with no requiement for a FI certificate.
In order to exercise the privileges of an IRI on ME aeroplanes you will also need either a CRI or TRI qualification as well.
As ab inition ME/IR instruction will be given on a MEP Class aeroplane the CRI qualification is more relevant.
FCL.915.CRI CRI — Prerequisites
An applicant for a CRI certificate shall have completed at least:
(a) for multi-engine aeroplanes:
(1) 500 hours flight time as a pilot on aeroplanes;
(2) 30 hours as PIC on the applicable class or type of aeroplane;
So do you have 30 hours MEP? You will also require a valid SP ME/IR
The next question is where are you likely to get the opportunity to use the qualification? To teach in an Integrated ATO you would need to hold an FI certificate as the candidates are ab-initio, leaving only a few modular schools to use your skills.
Any credit towards getting the FI if you have gained many hours as IR/ME instructor?
The only credit is the 25 hours teaching and learning which is common to IRI; CRI and FI Courses.