The ending of the Gospel of Mark has been a source of debate and confusion for a number of years. Some claim the last 12 verses are late and not original to Mark. It is true that some Greek manuscripts do not include this ending, and that some of them are early manuscripts. It does not follow that the ending is late and/or spurious.
Irenaeus makes a direct reference to it to the ending of Mark, and this reference is to the long ending. He quotes the Scripture (Mark 16:19 So then after the Lord had spoken unto them, he was received up into heaven, and sat on the right hand of God, KJV) and places the words at the end of Mark’s Gospel. This can be found in Against Heresies, with an estimated date range of AD 175-185.
Irenaeus, Against Heresies, Bk. 3, Ch. 10.5
“Wherefore also Mark, the interpreter and follower of Peter, does thus commence his Gospel narrative: The beginning of the Gospel of Jesus Christ, the Son of God; as it is written in the prophets, Behold, I send My messenger before Your face, which shall prepare Your way. The voice of one crying in the wilderness, Prepare the way of the Lord, make the paths straight before our God. Plainly does the commencement of the Gospel quote the words of the holy prophets, and point out Him at once, whom they confessed as God and Lord; Him, the Father of our Lord Jesus Christ, who had also made promise to Him, that He would send His messenger before His face, who was John, crying in the wilderness, in the spirit and power of Elias, Prepare the way of the Lord, make straight paths before our God. For the prophets did not announce one and another God, but one and the same; under various aspects, however, and many titles. For varied and rich in attribute is the Father, as I have already shown in the book preceding this; and I shall show [the same truth] from the prophets themselves in the further course of this work. Also, towards the conclusion of his Gospel, Mark says: So then, after the Lord Jesus had spoken to them, He was received up into heaven, and sits on the right hand of God; confirming what had been spoken by the prophet: The Lord said to my Lord, Sit on My right hand, until I make Your foes Your footstool. Thus God and the Father are truly one and the same; He who was announced by the prophets, and handed down by the true Gospel; whom we Christians worship and love with the whole heart, as the Maker of heaven and earth, and of all things therein.”
Translated by Alexander Roberts and William Rambaut. From Ante-Nicene Fathers, Vol. 1, edited by Alexander Roberts, James Donaldson, and A. Cleveland Coxe, Buffalo, NY: Christian Literature Publishing Co., 1885
Shorter excerpt from a different translation:
“Again, in the end of his Gospel Mark says, So the Lord Jesus, after He had spoken unto them, was taken up into Heaven, and sitteth at the right hand of God: confirming what is said by the Prophet, The Lord said unto my Lord, Sit on My right hand, until I make Thine enemies Thy footstool.”
Five Books of S. Irenaeus, Bishop of Lyons, Against Heresies, Translated by John Keble, London: James Parker and Co., 1872, p. 229
“It is probable that Justin Martyr, at the middle of the second century, knew this ending; in any case, Tatian, his disciple, included it in his Diatessaron.”[i] (The Text of New Testament, Its Transmission, Corruption and Restoration, 4th Edition, by Bruce M. Metzger, Bart Ehrman, Oxford: Oxford University Press, 2005, p. 323)
[i] Papias and Tertullian may have also referred to it; E.g. Tertullian, Against Praxeas, Ch. 2.1 “...we believe Him to have suffered, died, and been buried, according to the Scriptures, and, after He had been raised again by the Father and taken back to heaven, to be sitting at the right hand of the Father, and that He will come to judge the quick and the dead...” (Translated by Peter Holmes. From Ante-Nicene Fathers, Vol. 3. Edited by Alexander Roberts, James Donaldson, and A. Cleveland Coxe. (Buffalo, NY: Christian Literature Publishing Co., 1885)