Explanation/Reference:
See Note below.
Use the Shell to configure IRM for Exchange Search
This example configures IRM to allow Exchange Search to index IRM-protected messages.
Set-IRMConfiguration -SearchEnabled $true
Note: By default, the SearchEnabled parameter is set to $true. To disable indexing of IRM- protected messages, set it to $false. Disabling indexing of IRM-protected messages prevents them from being returned in search results when users search their mailbox or when discovery managers use In-Place eDiscovery.
Reference: Configure IRM for Exchange Search and In-Place eDiscovery
https://technet.microsoft.com/en-us/library/gg588319(v=exchg.150).aspx