The following comes from Catholic Online:
The writings of St. Irenaeus entitle him to a high place among the fathers of the Church, for they not only laid the foundations of Christian theology but, by exposing and refuting the errors of the gnostics, they delivered the Catholic Faith from the real danger of the doctrines of those heretics.
He was probably born about the year 125, in one of those maritime provinces of Asia Minor where thememory of the apostles was still cherished and where Christians were numerous. He was most influenced bySt. Polycarp who had known the apostles or their immediate disciples
Many Asian priests and missionaries brought the gospel to the pagan Gauls and founded a local church. To this church of Lyon, Irenaeus came to serve as a priest under its first bishop, St. Pothinus, an oriental like himself. In the year 177, Irenaeus was sent to Rome. This mission explains how it was that he was not called upon to share in the martyrdom of St Pothinus during the terrible persecution in Lyons. When he returned to Lyons it was to occupy the vacant bishopric. By this time, the persecution was over. It was the spread of gnosticism in Gaul, and the ravages it was making among the Christians of his diocese, that inspired him to undertake the task of exposing its errors. He produced a treatise in five books in which he sets forth fully the inner doctrines of the various sects, and afterwards contrasts them with the teaching of the Apostles and the text of the Holy Scripture. His work, written in Greek but quickly translated to Latin, was widely circulated and succeeded in dealing a death-blow to gnosticism. At any rate, from that time onwards, it ceased to offer a serious menace to the Catholic faith.
The date of death of St. Irenaeus is not known, but it is believed to be in the year 202. The bodily remains of St. Irenaeus were buried in a crypt under the altar of what was then called the church of St. John, but was later known by the name of St. Irenaeus himself. This tomb or shrine was destroyed by the Calvinists in 1562, and all trace of his relics seems to have perished.