Iris and Aerie are a duo of flying eyeballs. The duo has lower health when compared to other bosses, however both will split into two smaller versions of themselves. The smaller versions of the boss appears to have half the health of its larger flying eyeball cousin. If you apply poison to the larger eye, the poison will be applied when it splits to the smaller eyes.
It is also important to note that each eye the player kills will drop one boss worth of loot. This mean the player will receive the equivalent of 6 bosses of loot upon successful completion of this fight.
It is effective to shoot them from a distance and focus on one eye at a time. Split one of them and then focus on one of the smaller eyes, then the other one. Repeat this for the second eye.