I believe that the Iranian revolution, Iran/Iraq war of the 1980's, Hussein's leadership of Iraq, al qaeda, and ISIL  has left relations between the countries tepid at best. The leader of Iraq's Shiites, Al-Sayyid Ali al-Husseini al-Sistani, is his own man, and never has or ever will be a toady to Iran's Ali Khamenei.  These are countries with mutual interests and the same problem, powerful and distinct religious minorities causing insurrection.  ISIL was a threat right on the borders of Persian/Shiia culture and religion.  It is my understanding that Iran would prefer to deal with ISIL on its own and without U.S. and Russian assistance.  Arguably, the U.S. and increasingly Russia are in a quandary of what would happen should they withdraw.  Many think, myself included, this would be a war unseen in human history in relation to magnitude, focused on the schism in Islam.