Book 4, Chapter 34, Section 7 https://babel.hathitrust.org/cgi/pt?id=njp.32101074938968;view=1up;seq=231.
Often mistranslated as "The glory of God is man fully alive" (see http://www.touchstonemag.com/archives/article.php?id=25-05-003-e).
The context of the passage https://web.archive.org/web/20170126222027/http://earlychurchtexts.com/public/irenaeus_glory_of_god_humanity_alive.htm is: "And for this reason did the Word become the dispenser of the paternal grace for the benefit of men, for whom He made such great dispensations, revealing God indeed to men, but presenting man to God, and preserving at the same time the invisibility of the Father, lest man should at any time become a despiser of God, and that he should always possess something towards which he might advance; but, on the other hand, revealing God to men through many dispensations, lest man, falling away from God altogether, should cease to exist. For the glory of God is a living man; and the life of man consists in beholding God. For if the manifestation of God which is made by means of the creation, affords life to all living in the earth, much more does that revelation of the Father which comes through the Word, give life to those who see God."