International Race Of Champions is the true 'meaning'.
As for the Z in Z/28, (Or IROC-Z) as I recall, z/28 was the RPO code for the performance package on the old Camaro. People quickly must have learned that, so they made it into the model name.
Edit: The IROC Package was a performance upgrade over Z/28 for Camaros in 85-90. (Dodge had an IROC Car around the same time too actually...)
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