As the Black and Tans were disembarking outside the stadium, some took it upon themselves to open fire, causing chaos, said Foley. "They had gone to carry out searches, but took it as an opportunity to get revenge."

The only shots fired by the army were by an armored vehicle outside the ground, into the sky, in an attempt to get crowds to go back, Foley said.

Daragh Minogue is an academic who specializes in Irish sporting identity.

He said Bloody Sunday, and the subsequent myth, were defining events in Irish history.

"This is not what is supposed to happen at a sporting event, so it played a huge role in the story Ireland told itself," he said.

"Anything that fitted the national grievance against empire was included and inconvenient facts were forgotten, and that took generations to fade. The chaos that followed pretty much ensured Britain would leave the island," Minogue said.

In 2007, one of the most politically-charged moments in Croke Park's history saw the England rugby team play there, with national anthem God Save the Queen sung before the match, something unthinkable even a few years before. It was respected and politely applauded, before decades of emotion burst out as Ireland crushed England 43-13.

Four years later, the Queen herself visited Croke Park, and in a hugely symbolic speech, said "with the benefit of historical hindsight, we all see things we wish had been done differently, or not at all".

"That spirit is where we need to go with Bloody Sunday," said Foley. "It should never have happened, but if you delve into the stories of the people and what brought them there, it gives an understanding of what triggered that atrocity, and understanding creates space for healing."

Brexit coinciding with the centenary is another reason it is vital the truth is known, Foley said.

"Brexit is no reason for Britain and Ireland to drift apart," he said. "If there's a lesson to be taken from Bloody Sunday, it's that events are about people. You must understand where both sides are coming from, and find a middle ground. Otherwise, what is the alternative? We've lived through that already, we don't want to do it again."