Iran was called Persia not long before, hence the word Persian can be legally used for nationality, meaning Iranian. Persian could also refer to the language spoken mainly in parts of Iran/Tajikstan/Afghanistan -- and each variant of Persian can be called Farsi/Tajiki/Dari -- And, the same word could be used to refer people to speak the language
persian was the term mostly used before iranian revolotion on 1979,as it recalled persian golf,iran was chosen as revolotinaries didn't like persia as it reffered back to kings of persia in the past,but both
are used for the same country.
Persians are those who speak Farsi as their mother language. "Iranian" term covers all the groups and tribes like Kurd, Azari, Arab, baloch , turkman and so on.