Although bishop of Lyons in Gaul, Irenaeus (ca 130-208) was a Greek-speaking native of Asia Minor, a disciple of the venerable martyr Polycarp of Smyrna, who was in turn a follower of John the Evangelist.  So he was steeped in the theology of the Christian East, which was already at this early stage differentiating itself in noticeable ways from the spiritual teachings of the West.  One of these was the Incarnation.  Western theology tends to stress the necessity of Jesus assuming humanity so that he could atone for our sins.  The Christian East, by contrast, emphasizes that the Incarnation is primarily about God uniting in love with his creation.  This is evident in the writings of Irenaeus, even when he is passionately inveighing against Gnosticism and other heresies.  As Olivier Clement puts it in his invaluable The Roots of Christian Mysticism (New City Press 1993), "...Irenaeus developed a vigorous theology emphasizing the reality of the incarnation (and therefore of the flesh), the unity of the two Testaments, and the positive nature of history.  The word and the Holy spirit are the 'two hands of the Father'.  With them he creates, directs, attracts and fulfills humanity.  History appears thus as an immense procession of incarnation...God wishes to deify human beings but without destroying their freedom.  Time enables 'man to grow used to receiving God and God to grow used to dwelling in man'.  Irenaeus does not dramatize the fall" (pp344-345). 

Below are three passages of Irenaeus dealing with the Incarnation, all taken from his main work Against Heresies (pp36-38 in Clement).

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The Lord has given us a sign "as deep as Sheol and as high as heaven", such as we should not have dared to hope for.  How could we have expected to see a virgin with child, and to see in this Child a 'God with us' (Isaiah 7:  11-14) who would descend into the depths of the earth to seek for the lost sheep, meaning the creature he had fashioned, and then ascend again to present to his Father this 'man' [humanity] thus regained?

How could the human race go to God if God had not come to us?  How could we free ourselves from our birth into death if we had not been born again according to faith by a new birth generously given by God, thanks to that which came about from the Virgin's womb?

This is the reason why the Word of God was made flesh, and the Son of God became the Son of Man:  so that we might enter into communion with the Word of God, and by receiving adoption might become Sons of God.  Indeed we should not be able to share in immortality without a close union with the Immortal.  How could we have united ourselves with immortality if immortality had not become what we are, in such a way that we should be absorbed by it, and thus we should be adopted as Sons of God?