I highly doubt iriminage is much of a finishing technique. Irimi tsuki, or 'face thrust,' which is basically a stiff arm to the face that you step through, has been known to cause serious injury...perhaps that, but I doubt against a trained fighter it would do little more than crack a tooth or break a nose. Regardless, if an ovbiously aikido technique like shomen iriminage won a main card at UFC a serious shitstorm would happen. The rebuttal of which would consist of "well it took a real MMA fighter to make aikido work!" LOL.
According to aikido lore, shihonage is the most dangerous (historically) of techniques as one or two people have died from it. That being said, I would poop myself a little if someone could pull that off in the octagon.