There's an Arab population in Iran, just as there is in Iraq. It happened when Muslims conquered them and Islam subsumed their cultures in order to creat the foundations of the pseudo culture of Pan Arabism. Neither country is an Arab country.
Only 2% of Iranians are Arab - border of Iraq. Iranians and Iraqis look totally different and speak different language. Only similarity is the name and religion.
Iran isn't an Arab country, but Iraq is. After all, when the Arab Moslem armies subjugated it, not only the soldiers settled there but entire Arab tribes as well. The Shammar tribe is one of those tribes, and it's members consist of about 11 million Iraqis today.