"The Irish were slaves!"

was shouted at me in the library yesterday.
I actually had not said a word to this man. I was collecting an item I had on hold at the time.
Thankfully a male member of the library staff was just walking in from his lunch break. He saw what was happening and intervened. The shouting man was escorted from the library by another reader and the staff member. 
We could hear him still shouting outside. By then he was down in the creek which runs through the park next to the library. Someone had called the police and, by the time I left the library, they were there with the thankless task of trying to calm him down.
Yes, he was having an "episode" and will probably spend some time in secure accommodation again.
But he continues to tell me that the Irish were slaves. Back in the dim dark mists of the very early history of Ireland - prior to the 5thC - there were some slaves. Slavery was common in Europe at the time - although it was not recognised as such. People depended on it for their very survival. 
The modern day "slavery" myth and the claim that half a million Irish were killed by the British arose out of an article that was written by an unknown person and posted on the internet. This person called himself "John Martin" and he made some ridiculous claims, none of which were true. It was an act of extreme mischief making, designed to do harm. Like the "vaccines cause autism" nonsense it has been repeated over and over again by people who want to believe it. 
At teacher training I was taught the version of history that told me the English were entirely to blame for the problems facing the Irish. Nobody mentioned the words "potato famine", nobody said a word about the attempts to provide the starving Irish peasants with grain for bread when the potato crops failed. The English were not perfect and the lecturer made sure we knew it but slavery was never mentioned. Indentured workers were mentioned - and the role of the Irish upper classes in sending some of their tenants off to America as just such workers.
The man in the library is not normal. Those of us who know him know that. There are  however many "normal" people around who do believe the nonsense he was spouting. They believe it because they want to believe it, because it is convenient to believe it. 
I try to go hunting for actual facts when I come across something like this, especially when something niggles at me and suggests, "That might not be right."
It is like the photograph that appeared again and again on Twitter yesterday. It was either taken, enhanced or doctored in such a way as to make it seem that the Prime Minister was not adhering to the rules of "social distancing". I could not find another photograph but it still seemed unlikely that the Prime Minister would be blatantly ignoring something like this. Last night our international news service showed some footage of the same story. It was then clear that the Prime Minister had in fact been adhering to the rules. The photograph repeated so many times had in fact been designed to deceive.
I wonder how many other times this happens? How often are we taken in, especially when - like yesterday - a supposedly reputable news service and journalist does something designed to deceive us for political or personal gain?