Assuming a researcher is currently employed by an International European Interest Organization IEIO (e.g., CERN) and would like to come to a Host Institution based in the same country (e.g. University of Bern, Switzerland), he or she is not eligible due to mobility rule.
The other way, round, however, is indeed possible: A researcher now employed at an Institution in one country (e.g. University of Bern, Switzerland) will be eligible to go to an IEIO in the same country (e.g. CERN) as the future host institution.