One of the root causes of the conflict is the Sunni-Shiite rift in the Middle East. You could say that the main interest of Iran is to curb ISIS. The interest of Iran goes in Lebanon to the Hezbollah, in Syria to the Alawites. Iraq is mainly Shiite, and their explicit aspirations are to dominate what they call the Persian Gulf. So for them, that is the real threat embodied by ISIS, and for them to defeat the Islamic State is the main priority, so they are going to be involved in curbing, stopping ISIS, and they will do their utmost to do that.
On December 9th, 2014. See this statement in context.