An IRO simply mirrors the content of the Main domain. These are best utilized for multiple domain TLDs such as having .org, .net, .cc and .ws all show the exact same content as .com.
What it sounds as though you want would be IRMs which allow a different domain name to show entirely different content than the main domain. There are a limit of 5 IRMs per full package.
I would suspect you have already read this page but just on the off chance...
Hope this helps,