I don't know much about this but if I saw Irmscher pedals one ebay, and if i wanted to buy them, I'd have to make myself absolutely certain they're genuine!
If you are going to fit them I would have thought the old rubbers will need to come off, then a good clean of the pedal itself. Why don't you offer the Irmscher part upto the pedal to see of the screw holes are ok? If it is genuine there should be no problem.
I'd be very careful in fitting as you wouldn't want to compromise the level of control you have over the car by reducing operability of the pedals, by adding a cosmetic part. Also if you get half way through and change your mind, you could end up with drilled pedals with no rubbers on them, which it'd probably be best to fit new rubbers, instead of old ones (is it a legal/MOT requirement for pedal rubbers? Could lack of rubbers/modification to pedals affect insurance as it is safety critical? I don't know but they are points to bear in mind)
A friend of mine once had aftermarket metal pedals screwed to his pedals, they were terrible, loose, and you couldn't get proper control of the car. I refused to drive it!
HTH