Against Heresies does not survive in its original Greek except in fragments and quotations. What survives entire is a very literal Latin translation, wooden enough that scholars can often reconstruct the Greek behind it, plus an Armenian version of part. For seventeen centuries its first book, a systematic description of the Valentinian teaching of Ptolemaeus with its thirty aeons and its fallen Sophia, was effectively the only detailed account of second century Gnostic teaching that anyone had, and it was written by a man whose purpose was to refute it.
The discovery of the Nag Hammadi codices in Egypt in 1945 put primary texts in front of readers for the first time. The result was mixed for his reputation and interesting either way. His reporting turned out to be substantially accurate, and one of the recovered works corresponds closely enough to a system he describes that his summary can be checked against it line by line. His framing, that these were late innovations by named individuals departing from a single earlier faith, is what the recovered library made harder to sustain, since it shows a wider and less orderly range of second century Christian writing than his account implies.
He is also remembered for a use of his authority that his polemics do not predict. When the bishop of Rome moved to cut off the churches of Asia over the date of Easter, Irenaeus wrote urging him not to, on the ground that the difference in practice was old and had not previously broken communion. Eusebius, who preserves the letter, notes that he lived up to his name, which means peaceable.