Before they left the city, the crew filmed other IRA operations including a prolonged gun battle on the edge of Free Derry.

The gunmen are unmasked and appear utterly oblivious of the camera. It suggests a carefree attitude to the organisation’s own security. 

Yet we learned the IRA was very serious about minimising some risks attached to the project.

One of the crew recalled a conversation between McGuinness and an IRA member who was responsible for getting the team and their undeveloped footage out of Northern Ireland.

McGuinness is reported to have said: “You know if you don’t get that film across the border I’ll have to shoot you myself.”

The film did get across the border, but the IRA’s concern about keeping its contents secure didn’t stop there.

They had made a secret deal with the filmmakers giving them control over what would be said and who would be shown in the final film.

In New York we found legal papers that spelled out the arrangement.

The IRA had sought and obtained an agreement with Bowyer Bell for “control and rights of censorship”.

Bowyer Bell’s lawyer wrote that because the IRA leaders had been “cooperative and permitted the film to be made they could not be denied these rights,” nor could Bowyer Bell “permit these men to be compromised”.

Jacob Stern had first alerted us to this arrangement and outlined how the IRA intended to enforce it.

“They said, if any separate parts of the film were attempted to be taken separately to America, that we would be all shot at the airport. Just like that: ‘We’re gonna kill ya.’”

The film’s raw footage was never to leave Ireland.

Only the finished film could leave.

Stern explained that was why he had been brought to Ireland.

The IRA had insisted everything, including the music and the sound mix, was done before the film left the island. 

So the composer, who normally would do his work in a New York studio, had flown across the Atlantic.

The IRA laid on more filming in Belfast.

Another gun battle is captured in the west of the city.

And another bomb is filmed from start to finish.

Again, many of those filmed are easily identifiable.

“The film is propaganda, but it’s also a remarkable film,” said Diarmaid Ferriter, professor of Modern Irish History at University College Dublin. 

“The intimacy of the footage is really striking. There’s nothing else like that, that I’m aware of and to see what is being depicted in that film in 1972 in particular is quite remarkable, because of the level of access that’s there when it comes to senior IRA figures, but also the personal testimony of women in particular, which again is highly unusual in the context of republican paramilitarism and the history of the IRA.”

The red-haired woman featured throughout: patrolling with a rifle in her hand, driving a scout car for other bombers, taking notes at a meeting of the IRA’s Belfast Brigade – but also cooking dinner and actually going to school.

She turned out to be 17-year-old Geraldine Hughes. She died in 2006.

But her appearances throughout the film teach us to continue to be careful with what we’re seeing.

In that opening bomb sequence, the handbag containing the bomb changes shape before the bomb is planted.

The filmmakers were sometimes prepared to create a narrative that didn’t adhere to the truth.

The camera rolled south of the border too.

The film features a bomb-making class. The students mostly keep their backs to the camera, but the instructor is shown full face.

It took a bit of digging but we discover his identity.

His name was Paddy Ryan.

Extraordinarily he was then a member of the IRA’s Army Council.

And here he is, on camera, in his own Dublin home, taking IRA recruits carefully through the dos and don’ts of priming a bomb.

It is a truly remarkable scene in an already astonishing film.