Greckie oryginały jego najważniejszego dzieła, w 5.księgach, zaginęły. Zachowały się ich przekłady na łacinę zatytułowane "Adversus haereses" (Przeciw herezjom). Powstały w latach 180-185. Polemizował w nich z gnostycyzmem. Wiedzę na temat gnostyków czerpał z dzieł, które zaginęły. W roku 1526 "Adversus haereses" wydał Erazm z Rotterdamu. Współczesne wydanie krytyczne zostało dokonane w ramach serii Sources chrétiennes.

The full title of the principal work of Irenæus, as given by Eusebius (Hist. Eccl., v. 7), and indicated frequently by the author himself, was A Refutation and Subversion of Knowledge falsely so called, but it is generally referred to under the shorter title, Against Heresies. Several other smaller treatises are ascribed to Irenæus; viz., An Epistle to Florinus, of which a small fragment has been preserved by Eusebius; a treatise On the Valentinian Ogdoad; a work called forth by the paschal controversy, entitled On Schism, and another On Science; all of which that remain will be found in our next volume of his writings. Irenæus is supposed to have died about a.d. 202; but there is probably no real ground for the statement of Jerome, repeated by subsequent writers, that he suffered martyrdom, since neither Tertullian nor Eusebius, nor other early authorities, make any mention of such a fact.

As has been already stated, the first printed copy of our author was given to the world by Erasmus. This was in the year 1526. Between that date and 1571, a number of reprints were produced in both folio and octavo. All these contained merely the ancient barbarous Latin version, and were deficient towards the end by five entire chapters. These latter were supplied by the edition of Feuardent, Professor of Divinity at Paris, which was published in 1575, and went through six subsequent editions.