I've retrieved IR master form data like so:
Domain model:
For an ItemRevision, use a microflow to ExpandGRMRelationsForPrimary with Relation__/relationTypeName='IMAN_master_form'.
Set business object mappings:
'ItemRevision Master=MyModule.ItemRevision_Master' (note the space in the name)
This will give you a list of ModelObjects, one of which is an 'ItemRevision Master' form with user_data_1, user_data_2 and user_data_3