After you enable IRM for Exchange Online, administrators can use either of two methods to apply IRM protection to email messages:
Transport protection rules can be used to automatically apply IRM protection in Outlook and Outlook Web App. Administrators configure the transport protection rule action to apply an RMS rights policy template to messages that meet the conditions of the rule. The RMS rights policy templates are available to use with the transport protection rule action called Apply rights protection to message with. In the Exchange admin center this can be found in mail flow, under rules, then Apply rights protection to messages.
Administrators can create Outlook protection rules to automatically apply IRM protection to messages in Outlook