I tend to think of them as Angels/Elementals of anger/hatred, in the service of Aeron, the god of body/blood/rage. I usually think of Aeron and Camullos as allies, so they may serve him too.
Prior to the introduction of the summon, Ira were mentioned in the Odio's Prison entry. Odio was commanding the Army of Ira when Aeron withdrew his aura and allowed Kilmorph to pull him into the earth. Perhaps the Ira were upset by this, and defected to Camullos or just went off on their own. (There may not be much of a difference between the two options, as most of Camullos's angels are rebelling against and trying to usurp him. The Avatar is Camullos's archangel though, not Aerons.)