When I first became interested in studying second century Christian theology, one of the first works I read were those of Irenaeus.
Irenaeus' writings are probably the most difficult Christian writings in the entire patristic period to read. At the best of times he borders on incoherency. He uses some very unclear terminology. He makes prolific use of vivid imagery and totally fails to distinguish between the literal and metaphorical. Irenaeus has some very strange ideas that are not evidenced in other Christian sources from this period. He sees a strong connection between himself and the apostles, yet he gets extremely basic facts about the life of Jesus wildly wrong. Most patristics scholars have taken the same unflattering view on Irenaeus as myself. It was particularly popular to slander Irenaeus in the first 20 years of the 20th century, and though more people seem to like him now, I'm not convinced that this is for any good reason.
My frank advice therefore to anyone interested in studying second century Christianity is to avoid Irenaeus. Start instead with Justin Martyr's Apologies, and the Apostolic Fathers. Then read absolutely everything else from the second century, except Irenaeus. Then move to Origen and Clement of Alexandria in the early third century, and only then, and only if you really really must, go back and read Irenaeus.
I mention this because two blogs I read are starting a series of posts about one of Irenaeus' works. See here and here. I will read with great interest what they have to say about Irenaeus' theology. Because despite having a passing-fair knowledge of early Christian theology and having read Irenaeus' works a few times, I fully confess that I don't understand Irenaeus. From their posts so far it looks like John Behr's translation of Irenaeus' Apostolic Preaching is by far the best translation to read (I have not read it myself).