Born about the year 130AD St Irenaeus was a disciple of Polycarp who in turn was a disciple of John. Irenaeus was one of the great Apostolic Fathers, people who wrote the doctrine of the Church to explain to Rome that the Church was not a threat to the empire. He also was a great theologian who wrote the work, "Against the Heresies," one of the seminal writings of the second century which laid down clearly the foundations of subsequent doctrines. He was martyred about AD200.
As with yesterdays feast of St Cyril the first reading for Irenaeus is taken from Timothy in which Paul calls on Timothy to make sure he preaches and teaches only the true faith. The gospel is from John 17, the Farewell Discourse, where Jesus states his relationship with the Father. This was the main text Irenaeus used to combat the Gnostic heresy in his writing, which claimed that though Jesus was a deity, he was a terror deity that the Father.
Irenaues was important because he laid the foundation of much of what we believe today. His theological pedigree of Jesus->John the Evangelist->Polycarp of Smyrna->Irenaeus could not have been better. The many years John spent on Patmos reflecting on Jesus and the brilliant mind of Polycarp found a receptive student in Irenaeus.