Things such as hunger and pain are considered evil by some, however without hunger there would be no way to know that you were in fact hungry and without pain there would be no way to know that you have been hurt or injured. Therefore ‘evil’ is a way for humans to learn and evil also allows us to empathise with each other which lets us help others that need it; how could you help a hungry person if they did not know they were hungry?
In a way this is similar to parenting, the majority of parents will have to punish their children and those children might think that the punishment is ‘evil’ or unjust, however the parents will –usually- have a good motive behind the punishment, a good benevolent motive which is typically used as a lesson and is also not usually understood by the child as being a good thing for them later on. For example being punished for playing with fire, the child may think that this is unjust at the time as they hadn’t actually done anything they thought was ‘bad’ by setting fire to an anthill however the punishment was to help teach the child just as evil is used to teach humans morals.
Morals would also not truly exist if evil did not exist; if everybody did the right thing every single time, if everybody was forced to do the right thing by God, then all of our actions would be meaningless. Take, for example, an arranged marriage; in this scenario there tends to not be real love between the two people so the marriage is meaningless in the context of what a marriage is supposed to represent (love, unity, friendship, companionship) as forcing people to marry would not also force them to feel love.
In the same line of thought forcing people to always be good and do the right thing would not mean that those people actually know what good or the right thing is, they have not developed or earned that knowledge of good and evil for themselves. Irenaeus thought that anything given, not earned, would be next to worthless as you never actually learn anything.
Due to those two main point Irenaeus then argued that God is in fact doing a good thing by allowing what we perceive to be evil to exist as if there was no evil there would be no way for good to truly exist and there would also be next to no opportunities for people to learn.
That is the basis of Irenaen ‘soul-making’ theodicy, he said that evil was needed for the soul to be able to form and become ‘noble’, that we must make the choice to lead good lives and follow the way of God ourselves. Thus evil must exist for there to be a choice as, if there was no evil, there would be no choice and no way for the generic ‘human animal’ to change and advance into a ‘child of God’ which possesses morals.
So, yes, if one believes the bible (or any Abrahamic religion along with a few others) then Irenaeus’ theodicy would be able to adequately justify the existence of evil as a necessity for the growth of mankind both spiritually, mentally and morally.
However if you are from an agnostic or atheistic point of view then there are a number of flaws with the Irenaen theodicy which revolve around the concept and definition of God. If God is omnipotent, omniscient and omnibenevolent –which are all characteristics a being must be to be called a God, at least in Christianity- then logic would say that this God is capable of anything and should, therefore, be able to somehow create a world in which people never truly suffer and yet they can still know of suffering and learn from the knowledge of it.
A analogy for this would be that of a flower in a greenhouse, if you take the flower to symbolise humanity then a flower grown in a greenhouse would still be as beautiful and unique as a flower grown outside in the ravages of nature so surely if God is truly a God then it would be possible to create such a greenhouse world in which we, as a species, can grow and advance without the need for the hardships of nature (which would be replaced by evil in this scenario).
At the very least the amount of ‘evil’ and suffering in the world could be lowered or, as Swinburne argued, we could learn through pleasure more than through pain. As with the example before of learning about when to eat through hunger surely we could have learned when to eat from the pleasure of being full and the lack of pleasure when we are hungry or pleasure for committing good acts and a lack of pleasure for committing crimes or immoral acts.
Irenaeus also believed that everybody will go to heaven at some point; personally I believe that this could make sense if Irenaeus also said that –if you fail to get into heaven the first time- you are sent to retry life and you continue to retry life until you have learned from your mistakes. Alas Irenaeus in fact said that anything akin to reincarnation was ‘absurd’ and he instead believed in hell which, going off of his previous points, I believe is self-contradictory as hell would only offer a punishment and no true opportunity to learn from your mistakes.
In conclusion Irenaen theodicy can justify the existence of evil easily to religious followers however it runs into problems when people begin to question the existence or power of God.