On Irenaeus (who quoted 1 John 5:7) his quote provides early evidence for 1 John 5:7 since he mentioned he personally knew Polycarp, who knew the apostle John [who wrote 1 John] & te apostles:

From Irenaeus' Against Heresies (in Latin: Contra Haereses/Adversus Haereses), Book 3, Chapter III (3):
But Polycarp also was not only instructed by apostles, and conversed with many who had seen Christ, but was also, by apostles in Asia, appointed bishop of the Church in Smyrna, whom I also saw in my early youth, for he tarried [on earth] a very long time, and, when a very old man, gloriously and most nobly suffering martyrdom,' departed this life, having always taught the things which he had learned from the apostles, and which the Church has handed down, and which alone are true. To these things all the Asiatic Churches testify, as do also those men who have succeeded Polycarp down to the present time, — a man who was of much greater weight, and a more stedfast witness of truth, than Valentinus, and Marcion, and the rest of the heretics. He it was who, coming to Rome in the time of Anicetus caused many to turn away from the aforesaid heretics to the Church of God, proclaiming that he had received this one and sole truth from the apostles, — that, namely, which is handed down by the Church. There are also those who heard from him that John, the disciple of the Lord, going to bathe at Ephesus, and perceiving Cerinthus within, rushed out of the bath-house without bathing, exclaiming, " Let us fly, lest even the bath-house fall down, because Cerinthus, the enemy of the truth, is within." And Polycarp himself replied to Marcion, who met him on one occasion, and said, "Dost thou know me ? " [Polycarp replied], "I do know thee, the first-born of Satan." Such was the horror which the apostles and their disciples had against holding even verbal communication with any corrupters of the truth; as Paul also says, "A man that is an heretic, after the first and second admonition, reject ; knowing that he that is such is subverted, and sinneth, being condemned of himself.

(Irenaeus, Against Heresies, Book 3, chapter 3)
https://archive.org/details/antenicenefather01robe/page/416/mode/1up?

This is important since this links & connects back 1 John 5:7 from Irenaeus' quote of 1 John 5:7, to Polycarp, to the Apostle John, and is strong evidence of the Apostle John's authencity against critics' anti-1 John 5:7 claims. For John, who wrote 1 John 5:7, taught Polycarp, who then preached the Scriptures and taught Irenaeus:

And here's more on Irenaeus, Polycarp, and how this led to Irenaeus' quote of 1 John 5:7:

Irenaeus: "I ADJURE thee (1), who shalt transcribe this book,' by our Lord Jesus Christ, and by His glorious appearing, when He comes to judge the living and the dead, that thou compare what thou hast transcribed, and be careful to set it right according to this copy from which thou hast transcribed ; also, that thou in like manner copy down this adjuration, and insert it in the transcript.
...
(2)...For, while I was yet a boy, I saw thee in Lower Asia [Irenaeus is refering to Asia Minor] with Polycarp, distinguishing thyself in the royal court, and endeavouring to gain his approbation. For I have a more vivid recollection of what occurred at that time than of recent events (inasmuch as the experiences of childhood, keeping pace with the growth of the soul, become incorporated with it) ; so that I can even describe the place where the blessed Polycarp used to sit and discourse — his going out, too, and his coming in — his general mode of life and personal appearance, together with the discourses which he delivered to the people; also how he would speak of his familiar intercourse with John, and with the rest of those who had seen the Lord; and how he would call their words to remembrance. Whatsoever things he had heard from them respecting the Lord, both with regard to His miracles and His teaching, Polycarp having thus received [information] from the eye-witnesses of the Word of life, would recount them all in harmony with the Scriptures.