We don't have exact dates for Irenaeus, but many historians place his birth around 120AD, just twenty-five years or so after the death of John the last of the apostles. Tradition also suggests he either met or specifically studied under Polycarp (d.155), who in turn knew the apostle John.
Originally from Smyrna on the Aegean (in today's Western Turkey), Irenaeus found himself in the hinterland of Gaul, a Greek missionary to the Romanized Celtic population. Readers will recall that Gaul (roughly equivalent to today's France and the Low Countries) was conquered by Julius Caesar about fifty years before the birth of Christ and brought into the Roman Empire.
Irenaeus wrote of the persecution under the Emperor Marcus Aurelius in the 170's and in its aftermath he would become Bishop of Lugdunum (or today's Lyon).
Against Heresies (Adversus Haereses) is his best known work. Written about 180AD, Irenaeus tackled the growing problem of Gnosticism - a term used to describe a series of teachings which mixed Hellenistic philosophy, mystery religions, mythology, and Christian doctrine. While the New Testament hints of this in its nascent form during the time of the apostles, it did not become a major (or widespread) issue for the Church until the 2nd Century.
Until the discovery of the Nag Hammadi texts in 1945, Irenaeus was the major source for information on these groups. The book (still readily available) is a wealth of information, but admittedly a rather tedious read. The various Gnostic groups which he catalogs developed highly elaborate systems of thought that leave the modern reader frustrated and even bewildered. Over the course of the work, Irenaeus bears witness to much of the New Testament, quoting the apostolic writings (and testifying to Four Gospels) as authoritative Scripture. And thus in addition to helping us understand the state and formation of Christian doctrine, we have a witness to how the Early Church understood the Scriptures.
Another idea that comes into play (as already seen with Ignatius a generation earlier) is the idea of apostolic succession. The Roman Catholic Church twists these arguments to its own ends in order to justify its traditions and doctrinal innovations, but the purpose for Ignatius, Irenaeus, and others to make such an appeal is to counter the Gnostics who claimed to possess secret teachings handed down from the apostles.
Irenaeus countered them by demonstrating the pedigrees of contemporary bishops and tracing their ordinations and thus mentorships - right back to the apostles themselves. The implication is that the true doctrine (or Rule of Faith) has been preserved, belying the claims of the Gnostic teachers.
The Gnostics represent a spectrum of ideas but generally speaking they exhibited a hostility to the God of the Old Testament (a being they called the Demiurge) and the creation of matter. As such, they had a problem with basic New Testament concepts such as the Incarnation (God becoming man) or the Resurrection (the reconstitution of the body). They twisted New Testament teaching in order to justify their beliefs and often combined apostolic teaching with their own writings and conceptions of God. In some cases Gnostic teaching resulted in extreme forms of asceticism - rejecting meat as a form of reproduction (the creation of new matter). Others exhibited a kind of libertine ethic - mastering matter as it were, and this led to all manner of debaucheries and immorality.
Thus in order to counter them, Irenaeus provides a wonderful and helpful testimony to not just the Incarnation and Virgin Birth, but the notion that Christ is the Second Adam, as well as the doctrines of Resurrection and Judgment. He navigates the challenge of explaining how the Old and New Testaments relate - differing and yet constituting a unity with the same God as author. And thus we see in Irenaeus the earliest expression of a redemptive-historical approach to Scriptural interpretation with the Incarnate Christ being the focal point and essential to our salvation.
There are disputes regarding the date of Irenaeus' death and whether or not he was a martyr. The majority tradition is that he was martyred during the persecution instigated by Roman Emperor Septimius Severus around 202AD.
We do well to remember the testimony of Irenaeus, his defense of the faith as well as his martyrdom.