On the night of the 21st of August 1879 the Virgin Mary flanked by St Joseph and a bishop thought to be St John the Evangelist and an altar with a lamb and cross on it allegedly appeared on the gable wall of the Parish Church of Knock for a few hours. Fifteen people witnessed the vision including a child of five (page 60, The Evidence for Visions of the Virgin Mary).

The Archdeacon, Cavanagh, was in his house when this was going on.  His housekeeper Mary McLoughlin was the first to see something. 
Cavanagh's Lies
Cavanagh pretended that he didn't believe McLoughlin when she came to him about the vision at the gable and asked him to go and see it. Witness Mary Beirne testified in 1936, "Mary McLoughlin, housekeeper to Archdeacon Cavanagh, went to the parochial house to acquaint the parish priest, of the occurrence. He, however, did not visit the scene, believing, as he told his housekeeper, that it was a reflection from a stained-glass window erected some time before." She gave the same testimony during the 1880 interview with the Weekly News.
Cavanagh told McLoughlin that he thought it was a reflection from a stained glassed window knowing fine well he had no window that could do that or that resembled the apparition and it was dark. He wouldn't go to the apparition site as he feigned scepticism and disinterest.
Mary McLoughlin testified soon after the apparitions that when she told the Archdeacon of the vision "he appeared to make nothing of what I said." And the next day he heard "all about the apparition from the others who had beheld it; and then it came to his recollection that I had told him the previous evening about it, and asked him to see it." It is impossible to believe that the Archdeacon was that uninterested. He was acting.
Here is Cavanagh's 1880 account of what he was doing when he heard about the apparition. It was published with his approval by McPhilpin.
"As to the visions, the Archdeacon said, in effect: " On the night of the first Apparition my housekeeper asked leave to visit a friend, and remained out unusually late. While wondering what had become of her, she made her appearance in a very excited state, exclaiming : 'Oh! your reverence, the wonderful and beautiful sight ! The Blessed Virgin has appeared up at the chapel, with St. Joseph and St. John, and we have stood looking at them this long time. Oh the wonderful sight !' Inferring that the vision had disappeared, and omitting to question my housekeeper on that point, I did not go up, and I have regretted ever since that I omitted to do so. On another occasion a messenger was sent down to fetch me : I was in bed after a fatiguing day, and, having a prospect of hard work on the morrow, did not rise." — This manifestly appears as a triumph of the flesh over the spirit. — " I shall ever feel sorry that a sight of the Apparitions has been denied me, but God may will that the testimony to his Blessed Mother's presence should come from the simple faithful and not through the priests. Though I have not witnessed the divine manifestation I have seen the light, and once, when standing at some distance from the chapel, in company with others, a most brilliant star flashed along the gable, leaving a train of radiance."
It is impossible to believe him. His housekeeper Mary told him that she was not the only one who saw the sight. Thus he would have taken what she said seriously for that reason alone. He lies that he thought she said the vision had gone and that was why he never went to see the vision. Mary asked him to go to the gable according to her own account. He went to bed and another person came to ask him to go to the gable. Why did he not go then? And he was in bed and would have been in the dark and if there had been a light at the gable it would have been visible from his bedroom window in the dark. And he must have looked out the window at the gable at some point at least out of curiosity. The Archdeacon talked like a man who was playing the innocent. Also, nobody knows where he was when the vision appeared. And he claimed to be in his bed at the time when it vanished. How significant might that be?

There is no mention in his own account of his scepticism. He might have feared that if he pretended to think there was no vision and that was why he didn't go to the gable that it might be construed as evidence by sceptics that there was indeed no vision.
Another account of his is interesting. He was interviewed by a correspondent of the Weekly News of 14 February 1880. He said, "When my housekeeper returned home that night, she said that she had seen the Blessed Virgin at the chapel. At first, I gave no attention to her words but afterwards when I began to think that a wonder may really have been witnessed, I concluded that the people did not leave the church until the Apparition was visible no longer. Ever since, this has been to me a cause of the deepest mortification. But I console myself with the reflection that it was the will of God that the Apparition should be shown to the people, not the priest. If I had seen it, and if I had been the first to speak of it, many things would have been said that cannot now be advanced with any fair shadow of reason or probability on their side". Then the correspondent observed, "The strong emotion of the good pastor was so evident that both kept silent for some time."
It is impossible to believe that he didn't at least consider what she said to be true. He knew her well. If there had been a light at the gable with the figures inside it, why didn't she go to the window looking that way and point out? Why didn't she tell him to at least look out the window?
He said he thought those who were at the gable stayed until the apparition vanished. Again if he was so interested why didn't he stand at his back door and look at the gable?
He said that the gossips would have had a field day had he seen the apparition. He was evidently worried that people would suspect that he had had something to do with the apparition and was part of a hoax. He said this in 1880 after the testimonies of the witnesses had been recorded so evidently he considered them to be inadequate as proof that he only saw the apparition and nothing untoward was taking place.
He said that the credibility of the apparition would have been ruined if he had been the first to say that it had happened. What an odd thing to say? Why would he have to be the first to state that it happened? If people report a vision and you say you can't go to see it for you don't want to be the first to speak of it then clearly you are trying too hard to make an excuse. You would only try that hard if you knew what was really going on ie a hoax. And the Archdeacon DID make himself the apparitions first proper promoter! He started that as early as the next day and even took part in the Church investigation of the witnesses.
But that aside, why would people automatically blame him? Why would he assume they would? He is evidently concealing the fact that he knew more than he let on. And again if the evidence is good enough, it wouldn't matter who saw the apparition or who told about it.
The Archdeacon was always embroiled in enough controversy to know that it's best to let people say what they like. He was in the habit of doing so anyway. He had nothing to fear if he was not involved in a hoax. He talked like a man that had something to hide. He thought, "I am doing wrong by orchestrating this fake miracle and I will get found out if I am not careful." If you are overcautious in case you get accused of a hoax, then you are very probably guilty. If you assume people will think you did x if you do such and such, then you must have x on your conscience.
If the Archdeacon refused to go to the gable that night lest the credibility of the vision be diminished, that raises some questions. His reasons actually look like excuses.
The Archdeacon was silent and annoyed after he said the people would think he was behind the apparition hoax. That's important. And even more so when nobody accused him at all of being a hoaxer. That accusation has only been considered in recent decades. Poor guilty Cavanagh!

The light - if as bright as the witnesses said - would have shone in the windows at the back of his house. He must have noticed it. Even after been told about the apparition he pretended not to notice. The picture below shows the Archdeacon standing some distance in front of his house. The apparition gable can be seen just above his head. There was a clear unobstructed view of the apparition gable from his house.
Just above his hat, you can see the wooden planks used to support the gable as the people were stealing the cement thought to have magic powers.

If the Archdeacon had genuinely thought the apparition was an illusion or trick of the light he would have looked out. If you see a strange light you look out. His backdoor gave him a full clear view of the apparition gable as well. The Archdeacon knew more about the apparition than he was letting on. He was either conspiring with the witnesses to pretend that a vision had occurred or he had been involved in deceiving them.
He allegedly went to bed early that night and when the light was out he would have seen the apparition light shining in his bedroom window from which the gable could be seen.
The next day he set about promoting the vision and was even collecting rainwater that hit the gable into bottles to give to pilgrims. What a suspiciously fast conversion!
A priest who encouraged people to believe the mortar from the gable had occult powers could hardly be described as open-minded. He was biased in favour of the miraculous - that is, credulous. If the vision was a hoax, he was still able to make himself believe that it was real even if he was involved. People died from consuming the mortar (Knock The Virgin's Apparition in Nineteenth Century Ireland, page 245). Also the mortar being taken damaged the gable sacrilegiously. If some trickery had been afoot, say if something had been fastened to the wall to deceive the people, then it was to the hoaxer's advantage to have the wall vandalised and the evidence of tampering eliminated and lost. For example, evidence of nails having being used to attach the images to the wall would have disappeared. The man who encourages credulity is a man without real integrity. When one encourages the risk of being deceived one would deceive.
The Story of My Life by the Nun of Kenmare was published in 1891. "While I was in the church one day I saw a bright light above the altar, and all the people were exclaiming, "There it is! there it is! Now we have seen it for ourselves." I was somewhat impressed myself, and hoped that at last I had seen a supernatural sight, even if it was only a bright light. I was kneeling when I first saw the light, but when I rose up from my knees the light disappeared. I at once knelt down again, and lo, the light shone once more as bright as ever. I tried this experiment several times, and was then convinced that it was some reflection. I had made up my mind to investigate everything thoroughly when I came to know, though my prejudices were in favour of believing everything. I now went near the altar, and at once found out the cause of what seemed supernatural. It was simply a very large glass stone, which had caught the reflection of the setting sun. I dare don't touch anything about the shrine, so I went at once to Father Cavanagh, whose house was quite near, and asked him to come and remove the vision, for I thought it was dreadful to have the people deceived, But to my amazement - and I must admit to my indignation - he would not remove it. This made me very skeptical as far as he was concerned" (page 268 to 291).
Cavanagh knew that the apparition made it look like Mary agreed with his antics and his unfair opposition to the Fenians and the Land League. He encouraged that thinking. A man who wanted the evils of the Landlordism to be maintained and who claims the Virgin Mary to be as bad is simply not to be trusted. Would Mary appear if people were going to take that interpretation of her action?
The nun stated in her other works how she was so anxious to have a vision. Yet she was far from easy led in the above story. Thus we must consider her story to be true and reliable.
This story tells us a number of things.
The people were easily led - were the original visionaries as bad?
Cavanagh had a stone that was able to make the fading light of the sun become very bright. A stone like that could have been used to make the light of the original magic lantern used on the gable wall far brighter than usual.
Cavanagh was open to letting the people be deceived. He could have let the stone stay and tell them the light was an illusion but he did not. The nun was right to feel her "indignation".
When Cavanagh was priest of Westport in Mayo, he said that a stranger had given him the exact amount of money needed to build a facility for young girls. The priest met the donor at Lecanvey near Croagh Patrick (page 32, Venerable Archdeacon Cavanagh). It seems he did nothing to discourage the notion that the man was none other than St Joseph himself! The pious believed it was (page 32, ibid). It is obvious he wanted people to believe in visions.
In Venerable Archdeacon Cavanagh page 113 we learn that two men on the run went to the priest's house for the sacraments. A stranger opened the door and he seemed to know what they were there for. He told them the Archdeacon was tired and asked them to come back in the morning for Holy Communion and Confession. They told the priest the next day and he said that they must say nothing about the man until after he is dead. Pious belief was that the man was an angel. From what we know today, he could have been a gay lover! We know that the Archdeacon when dying wanted young Father Reidy, a Curate in Claremorris, around all the time! The Archdeacon was trying to create a supernatural mystery!
The Archdeacon knew that if an apparition is reported, people can come privately and pray but official pilgrimages are wrong. The Church has to approve the apparition before they can be allowed. This is Church law even today for the Church bans official and public pilgrimages to Medjugorje for it is not an approved apparition. Devious Cavanagh welcomed the first organised pilgrimage to Knock in March 1880. Fifty members from the Holy Name Confraternity came from Limerick. He gave an address to them that was printed in the Munster News (24 March 1880) which asserted that Mary appeared with Joseph and John at Knock. A formal presentation of the pilgrimage banner took place - the Archdeacon put it beside the altar. Later in 1880, the Cork pilgrimage arrived with a grand new altar for the parish Church! (page 79, Venerable Archdeacon Cavanagh). The Church looked very good in 1880 - a remarkable transformation from a plain church with few seats and a clay floor and flagstones in the sanctuary area and rough altar from the previous year!
Cavanagh was one of the three or four or five priests involved in the investigation of the apparition which took place 8 October 1879. Even pious writers devoted to the apparition and the shrine admit that this investigation was careless and unprofessional (page 174, Knock The Virgin's Apparition in Nineteenth Century Ireland). Yet it must be assumed that the commission never made a final decision as to the apparition being authentically supernatural or otherwise (page 177, Knock The Virgin's Apparition in Nineteenth Century Ireland). There is no evidence or record that it decided one way or the other.
The Archdeacon told the Weekly News of 14 February 1880 regarding his strange absence from the apparition at the gable, "If I had seen it, many things would have been said that cannot now be advanced with any fair show of reason or probability on their side." In other words, I would have been accused of fraud had I gone to see it. This is a bizarre thing to say. The Archdeacon had no concern for reason or probability for he accepted all the ridiculous miracle reports. It is staying away from the scene of the vision that should open the door to suspicion. More importantly, the Archdeacon was admitting that he was the target of rumour and insinuation. Clearly, there was a feeling among some that he was responsible for the fake miracle.  If he was not directly responsible he was responsible for muddying the truth after to twist things into making a case for the vision.  So the feeling is understandable.

And it is a feeling you should have now at the end of this article!