To apply IRM protection to a list or library, follow these steps:

1. Navigate to the SharePoint site containing the list or library to protect.
2. If it is a classic SharePoint site document library, click the Library tab and then click Library Settings. If it is a modern site document library, select the gear icon and select Library settings. Note: if you are applying IRM to a list, click the List tab and then click List settings:

3. Under Permissions and Management, select Information Rights Management.

4. Select the Restrict permissions on this library on download checkbox and add a policy title and description to be displayed in protected documents when they are opened. Click SHOW OPTIONS to display the list of possible restrictions and policies that can be applied:

5.  Choose one or more options to apply to the documents and click OK when you're finished:

6. Click OK to save the IRM settings.

The IRM settings will be applied when documents are downloaded from the library. For lists, IRM only applies to files that are attached or uploaded to the list items, not the list items themselves.