I’ve been working with IRM policies in Office 365 recently, in order to build a solution to prevent information leaks. Basically, Information rights management (IRM) is a subset of digital rights management (DRM), technologies that protect sensitive information from unauthorized access. You can choose which specific libraries you want to apply this kind of policies in your SharePoint Environment. You can restrict which kinds of actions can be taken on protected documents, preventing information to be forwarded and accessed by unauthorized people. If the documents with IRM/RMS protection are downloaded or forwarded to any place outside the SharePoint/Office 365 Environment, they can only be viewed only after the user authenticates with his credentials AND he has permissions on the source library.
At first, IRM/RMS in SharePoint libraries works out-of-the-box only with Office and PDF files. When it comes to SharePoint Online, the configuration is easier, because you can have your IRM policies easily activated with Azure RMS.
The definition is on the Office 365 administration level and not on the SharePoint 365 admin only. Here’s a quick step by step on how to activate it.