Migration of IRM (Information Rights Management) is not supported by ShareGate Migrate.
When you migrate Documents encrypted via IRM from an on premises version of SharePoint to Microsoft 365, your documents should be unencrypted and usable at the destination.
If you migrate documents encrypted via IRM from Microsoft 365 to Microsoft 365 however, your documents will remain encrypted and they will be unusable at the destination.
To avoid any issues, it is recommended to turn off IRM at the source before performing your migration.
You can set IRM back at the destination after your migration.