A donor region/country is a country from which migrants LEAVE when they emigrate/migrate.
A receiver region/country (also known as a host region/country) is a country to which migrants COME to live when they emigrate/migrate.
For example:
An Irish person emigrates to Australia from Ireland
Donor country: Ireland
Receiver country: Australia
similarly;
A person migrates from the west of Ireland to Dublin
Donor region: West of Ireland
Receiver region: Dublin