Short answers, pulled from the story.
Irenaeus was a Greek bishop in the second century who guided and expanded Christian communities in the southern regions of present-day France. He is best known for opposing Gnostic interpretations of Christian Scripture and defending orthodoxy, especially in his work Against Heresies, written around 180.
Against Heresies, whose full title is On the Detection and Overthrow of the So-Called Gnosis, is a five-book refutation of Gnosticism, particularly the teachings of Valentinus. In it Irenaeus offered three pillars of orthodoxy: the scriptures, the tradition handed down from the apostles, and the teaching of the apostles' successors.
Irenaeus is the earliest surviving witness to regard all four now-canonical gospels, John, Luke, Matthew, and Mark, as essential scripture. Before him, Christians disagreed over which gospel to prefer, with Anatolian Christians favoring John and Matthew being the most popular overall.
Irenaeus was born in Smyrna in Anatolia, now İzmir in Turkey, some time before Polycarp's death around 155 AD, and became the second bishop of Lyon. The date of his death is unknown but must have occurred at the end of the second or the beginning of the third century.
Pope Francis declared Irenaeus the 37th Doctor of the Church on the 21st of January 2022 in the Catholic Church. Francis also conferred upon him the supplementary title Doctor unitatis, meaning Doctor of Unity.
Irenaeus claimed authority through Polycarp, whom he had seen and heard preach in Smyrna, and who was said to have heard John the Evangelist. He used apostolic succession, the unbroken chain of bishops traceable back to the Apostles, to argue that his interpretation of Scripture was correct against the Gnostics.