Irenaeus' Against Heresies is available online at the wonderful ccel.org and the same translation is available in Schaff's Ante-Nicene Fathers (vol.1). It is well worth a few hours' time, though a warning to the reader: unless you are very keen to learn about second-cenutry Gnosticism, skip the first two books!

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For those who just want a taster of Irenaeus, John Behr's translation of The Demonstration of Apostolic Preaching is excellent, and comes with very helpful notes and introduction, highly recommended.

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For anyone interested in following up Irenaeus' main themes of incarnation and recapitulation, one outstanding introduction is worth mentioning: Gustaf Wingren, Man and the Incarnation: A Study in the Biblical Theology of Irenaeus.

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