He is the most famous Second century polemicist. He wrote "Against All Heresies" to refute the Gnostics.
In book One, he lays out their theology and teachings.
In book Two, he emphasized God's unity. The Gnostics believed in two gods, a bad material one and a good spiritual one. They separated God the Father and God the Son as different entities. Irenaeus shouted a big "NO". They Old Testament and New Testament God are one and the same and Jesus came in a real body.
Book Three, he refutes Marcion, who only believed in the Bible letters to the Greeks.
Book Four is more Scripture against Marcion.
Book Five is about the Resurrection of the body.
It's amazing how people still follow the Gnostic tendencies to despise physical bodies and to seek freedom from them.
Heresies are never new, and they always recycle into new systems. Gnosticism lives on in New Age, neo-Paganism, and the Emergent Church. It will probably recycle into something else tomorrow.
Beware of any belief that glorifies solitude. God never meant us to be separated from people and on our own.