It is enough for me to start this revelation by noting that one line is cited almost verbatim in Clement's Exhortation to the Pagans and Irenaeus's Against Heresies. As a result it would seem that Clement's invented proclamation of Jesus to the Gentiles was the source of the beginning of Irenaeus's account against the Marcosians. So we read first in Clement:
This Jesus, who is eternal, the one great High Priest of the one God, and of His Father, prays for and exhorts men. "Hear, ye myriad tribes, rather whoever among men are endowed with reason, both barbarians and Greeks. I call on the whole race of men, whose Creator I am, by the will of the Father. Come to Me, that you may be put in your due rank under the one God and the one Word of God; and do not only have the advantage of the irrational creatures in the possession of reason; for to you of all mortals I grant the enjoyment of immortality. For I want, I want to impart to you this grace, bestowing on you the perfect boon of immortality; and I confer on you both the Word and the knowledge of God, My complete self. This am I, this God wills, this is symphony, this the harmony of the Father, this is the Son, this is Christ, this the Word of God, the arm of the Lord, the power of the universe, the will of the Father; of which things there were images of old, but not all adequate. I desire to restore you according to the original model, that ye may become also like Me. I anoint you with the ungent of faith, by which you throw off corruption, and show you the naked form of righteousness by which you ascend to God. Come to Me, all ye that labour and are heavy laden, and I will give you rest. Take My yoke upon you, and learn of Me; for I am meek and lowly in heart: and ye shall find rest to your souls. For My yoke is easy, and My burden light." Let us haste, let us run, my fellowmen--us, who are God-loving and God-like images of the Word. Let us haste, let us run, let us take His yoke, let us receive, to conduct us to immortality, the good charioteer of men. Let us love Christ. He led the colt with its parent; and having yoked the team of humanity to God, directs His chariot to immortality, hastening clearly to fulfil, by driving now into heaven, what He shadowed forth before by riding into Jerusalem. A spectacle most beautiful to the Father is the eternal Son crowned with victory. Let us aspire, then, after what is good; let us become God-loving men, and obtain the greatest of all things which are incapable of being harmed--God and life. [Exhort. 12]
Just as Clement speaks - as a follower of St Mark - of the initiates 'uniting' and being 'yoked' with Jesus,
Irenaeus parodies the original material, now making 'Marcus' (apparently the spokesperson of this 'heresy') only interested in having sex with dupes of both sexes:
"I am eager to make thee a partaker of my grace, since the Father of all doth continually behold thy angel before His face. Now the place of thy angel is among us: it behoves us to become one. Receive first from me and by me grace. Adorn thyself as a bride who is expecting her bridegroom, that thou mayest be what I am, and I what thou art. Establish the germ of light in thy nuptial chamber. Receive from me a spouse, and become receptive of him, while thou art received by him. Behold grace has descended upon thee; open thy mouth and prophesy." [Against Heresies 1.13.3]
The key to the understanding, is not merely that virtually everyone recognizes that Clement (Strom 6.14) 'borrows' from the same 'Marcosian writing' which Irenaeus cites in AH 1.15, but now that Irenaeus parodies the ending of Clement's Exhortation.
Look carefully at the two opening lines again:
I want to impart to you this grace,
ἐθέλω καὶ ταύτης ὑμῖν μεταδοῦναι τῆς χάριτος
I am eager to make thee a partaker of my grace
μεταδοῦναί σοι θέλω τῆς ἐμῆς χάριτος,
The ever more likely scenario is shaping up, that Clement is not really citing Marcosian writings (as Schaff claimed) but rather that Clement is the source of Irenaeus's information about the followers of Mark.