Let's suspend our beliefs, whatever they are, for a moment, and look at the passage objectively. Irenaeus was discussing possibilities as to the name of Antichrist (which he considered to be an individual who would appear future to his time) and then made the following statement from his writing Against Heresies Book 5 Chapter 30:If you already believe that, it's easy to see it there. That's ceratinly what Eusebius believed. But I don't see it there.
We will not, however, incur the risk of pronouncing positively as to the name of Antichrist; for if it were necessary that his name should be distinctly revealed in this present time, it would have been announced by him who beheld the apocalyptic vision. For that was seen no very long time since, but almost in our day, towards the end of Domitian's reign.
What was it "that was seen...towards the end of Domitian's reign?
Was it not "the apocalyptic vision" from the previous sentence? And who was the one who had seen this vision? Irenaeus had been talking about the number of the Antichrist's name, namely "666", and speculating about a couple of names whose letters added up to that number. Only in the Apocalypse do we find any information about 666 being the number of The Beast (the personal Antichrist). Who, other than the author of the Apocalyse who wrote about his vision on Patmos, and about the number of the Beast being 666, could have "beheld the apocalyptic vision"?
So the bottom line seems to be that Irenaeus affirms that John received his "apocalyptic vision" toward the end of Domitian's reign.