It doesn't answer the question. If the disobedient are the sons of the devil then they are not the sons of God, thus they are not disinherited.
It answers it quite well for me after rereading the last several chapters of his Book IV and the whole chapter from which you quoted that bears this title. Especially the next sentence.
He makes the point that People are even called Sons of Satan from the womb (see his reference to David saying so within).
My point, which I believe follows his, is that he's not really talking about a converted (born again) Christian loosing their salvation but rather all the lost being "disobedient" and thus "disinherited" (i.e. un-saved). When you actually read all the other things he's talking about in the whole chapter and at least the one prior you find this out. When he says, 'disinherited', he means it much more broadly than as in a saved person who's been converted then turning around and loosing their inheritance for a second time. He's talking about the fall of man and even the fall of Satan and his angels by his use of 'dis-inheritance' and disbelief in this context.
I.e. Humanity being 'disinherited' at the fall. Thus our need to be "born again" in the first place.
Regardless, I don't really think he's talking about how someone becomes saved then looses their salvation, in my opinion.
Here's an example (one of many) throughout his Book IV of how he's using the phrase "dis-inherited":
Vain, too, is [the effort of] Marcion and his followers when they [seek to] exclude Abraham from the inheritance, to whom the Spirit through many men, and now by Paul, bears witness, that "he believed God, and it was imputed unto him for righteousness." And the Lord [also bears witness to him, ] in the first place, indeed, by raising up children to him from the stones, and making his seed as the stars of heaven, saying, "They shall come from the east and from the west, from the north and from the south, and shall recline with Abraham, and Isaac, and Jacob in the kingdom of heaven; " and then again by saying to the Jews, "When ye shall see Abraham, and Isaac, and Jacob, and all the prophets in the kingdom of heaven, but you yourselves cast out." This, then, is a clear point, that those who disallow his salvation, and frame the idea of another God besides Him who made the promise to Abraham, are outside the kingdom of God, and are disinherited from [the gift of] incorruption, setting at naught and blaspheming God, who introduces, through Jesus Christ, Abraham to the kingdom of heaven, and his seed, that is, the Church, upon which also is conferred the adoption and the inheritance promised to Abraham. (Chapter VIII)
Irenaeus is really using the term "disinherited" as a synonym for the "lost", "un-saved", "those outside the Kingdom of God", etc. He's NOT using it to teach once saved people are becoming un-saved. He doesn't think "dis-inherited" people were ever saved to begin with is my point. Now you can either evaluate this evidence and verify it in his book, or go on believing that it's a OSAS=no statement of Irenaeus' or not. It's up to you. But there's ample evidence that He's using this phrase in Chapter 41, the same way he does in Chapter 8.
It (the phrase you bolded) certainly does sound like he's taking a negative position on OSAS at first glance. However, I'm not convinced that's actually his point after re-reading his broader topic and the heresy that he was addressing in this book.
He's teaching
against a serpent seed type of heresy whereby the Marcions were teaching multiple gods (such as Satan being a literal father, etc.)