Vidqun, Since Justin never explained why he concluded the author was the same John as described as an apostle from the Gospel story it's hard to evaluate his judgement. How did he deal with the theological and linguistic differences clearly seen by others? However, if we are to conclude Justin's opinion on the matter is conclusive then ought we not regard his interpretation of the apocalypse as likewise authoritative? I doubt you do as his differs from yours most fundamentally
As to Irenaeus, he was said by Eusebius to have said that there was a "John also, the Lord's disciple, when beholding the sacerdotal and glorious advent of His kingdom, says in the Apocalypse:.". Although commonly this is assumed to be referring to the Apostle called John in the gospel, that is hardly certain. In the very context of the above quote, Irenaeus quoted repeatedly from the "gospel" attributed to the apostle John only to subsequently a few lines later introduce the "apocalypse" as a work by "John the Lords disciple". I find that a bit suggestive he understood the John who wrote the revelation as distinct from the gospel writer. He may also have simply used the term "disciple" as Eusebius (quoting Papias) otherwhere does in his Church History III:
4. If, then, any one came, who had been a follower of the elders, I questioned him in regard to the words of the elders — what Andrew or what Peter said, or what was said by Philip, or by Thomas, or by James, or by John, or by Matthew, or by any other of the disciples of the Lord, and what things Aristion and the presbyter John, the disciples of the Lord, say. For I did not think that what was to be gotten from the books would profit me as much as what came from the living and abiding voice.
5. It is worth while observing here that the name John is twice enumerated by him. The first one he mentions in connection with Peter and James and Matthew and the rest of the apostles, clearly meaning the evangelist; but the other John he mentions after an interval, and places him among others outside of the number of the apostles, putting Aristion before him, and he distinctly calls him a presbyter.
6. This shows that the statement of those is true, who say that there were two persons in Asia that bore the same name, and that there were two tombs in Ephesus, each of which, even to the present day, is called John's. It is important to notice this. For it is probable that it was the second, if one is not willing to admit that it was the first that saw the Revelation, which is ascribed by name to John.
7. And Papias, of whom we are now speaking, confesses that he received the words of the apostles from those that followed them, but says that he was himself a hearer of Aristion and the presbyter John. At least he mentions them frequently by name, and gives their traditions in his writings. These things, we hope, have not been uselessly adduced by us.
Note John the Presbyter (Elder) as distinct from John the Apostle is included among the "disciples of the Lord" and called the author of the revelation.