To put it simply, IAM is all about compliance (authentication + identity management + governance) within an enterprise security parameter with a population size usually less than 50,000. In fact, a figure of >30,000 workforce with partners are not usually seen.
IRM goes beyond an enterprise and it's about the Internet of Things. It is usually sized to support a population of more than a million. It's a totally different playing ground.
That's fun! :)