Hi Paul,
I’m sorry that the IRM within Office 365 setup doesn’t works for Office 2011 for Mac at present. If you want to protect your documents, I suggest you encrypt with password as a workaround.
Meanwhile, I invite you to submit feedback about this requirement to our product team. Many features of current programs were designed and upgraded based on customers’ feedback. We’ll appreciate your comments and suggestions to help improve our services.
Thanks,
August