Wow!
So, Irenaeus claimed to have seen Polycarp when he was a youth. But that does not mean he embraced, or even knew, all of Polycarp's teaching.
While I have not seen any major other treatise explaining why Irenaeus was a major heretic (actually, outside of Church of God circles, I have little that specifically considers him to have been a heretic, though groups like the
Jehovah's Witnesses probably do), it appears to me that he may have been the most dangerous heretic.
Why?
Because, Irenaeus' heresies were not obvious to those outside the true Church of God. His heresies and false statements were less numerous and less obvious than Justin's, thus have been missed by nearly all scholars (though some have noticed one or two errors he made).
What were his most important heresies?
Irenaeus heard Polycarp, yet made deals with Rome that disagreed with Polycarp's teachings. Irenaeus knew that Justin did not agree with Polycarp, yet he approved of Justin--and sadly seemed to prefer to be influenced more by him that by Polycarp.
Irenaeus knew that Polycarp condemned heretics such as
Marcion and
Valentinus, but failed to mention that they were still allowed to be Roman Catholic until at two decades later. Irenaeus supported the Roman Church even though Irenaeus knew they tolerated heretics that had earlier been condemned by Polycarp (and eventually by Irenaeus himself).
And..it gets worse as you continue to read that link...good find Willie..