I pronounce it EE-oh. I think both it's Greek and Japanese form are lovely and usable. The shortness of it means it's a first name only for me.
Io HyacintheIo ElviraIo MargalitIo ÉmilienneIo NicolasaIo Sylvestra
Io ThisbeIo UndineIo AtalantaIo Marianthe