'I don't want to be making films that people feel they have to go to because they feel they are good for them," the great Kim Longinotto tells me.

She need not fear. For nearly 40 years, the documentarian has been telling stories about people who, though often nudged to the edges of society, rarely come across as victims seeking our sympathy. The cracking Shinjuku Boys went among transgendered citizens of Tokyo. Sisters in Law dealt with women working in the Cameroonian legal system. Divorce Iranian Style did what the title claimed to do.

“It is all about telling stories,” she says. “If I can get you interested in a story then maybe you will learn something about yourself.”

Born in 1952 to an Italian dad and a Welsh mum, Longinotto does nothing to expunge clichés concerning those nations’ tendencies towards loquaciousness. She claims to have been a shy sort of kid, but she is the friendliest sort of adult. One can understand how she persuades people in difficult situations to come before the camera.

“Well, it’s actually not like that at all,” Longinotto says. “I find that people want to tell their stories. It’s often the opposite. They often force their way into the film.”

And she goes on to tell an extraordinary story about a young Kenyan girl she met while making The Day I Will Never Forget, her film on female genital mutilation. The kid insisted that Longinotto record her reading a poem about her own ordeal and her mother's agreement not to inflict that suffering on a younger sister. But would mum keep that promise?

“She said: ‘Of course she will. Because you recorded it, Kim. You’re a witness.’”

All of which goes some way to expressing the continuing power of the filmed image. After such rigorous adventures in the field, Longinotto must have viewed her latest project as something of a holiday. The gorgeous Love is All scores archival footage from British Film Institute and the Yorkshire Film Archive to tunes by Richard Hawley as it seeks to investigate how notions of love have changed since the birth of cinema.

“Oh yes, Donald, it was a pleasure it really was,” she effuses. “Editing is always my favourite part. But it was still stressful. We had so little time. Every day, we’d get hours more footage. And I normally work from about 15 hours of footage.”

Really? That doesn’t seem like much for the sort of observational films in which she usually specialises.

“I never understand why people shoot so much. Can you? I am throwing away stuff as I am filming.”

Playing as part of the Stranger than Fiction documentary season at the Irish Film Institute, Love is All comes at its subject from a number of angles. Information films, cheap melodramas and surprising news reels tells us about changing attitudes to race, sexuality and class. One of the most striking sequences, culled from Springtime in an English Village, sees a young Afro-Caribbean girl being crowned as "queen" at a seasonal festival somewhere in Norfolk during the middle of the century. Hers is just about the first black face we have seen in a largely chronological journey through the past.

“That is such an important section,” she says. “I love it because it is so English. You have the teacher kissing her. It’s such a British setting. If you look at films from the 1970s on that issue they are all a bit one-dimensional. It’s important to tell other stories.”

Yes, the film has much to say about intolerance. But it’s important to also talk about tolerance.

“There was some pressure to make it just about romantic love,” she says. “We wanted to show change. I went into this quite naively actually. We thought there’d be an obvious progression with women. We’d start in 1898. The corsets would go. Those clothes would go and the women would become progressively more liberated. But we found that film-makers were always ahead of their time. They were pointing to the future.”

Interestingly, it is not until the final 15 minutes that we encounter familiar mainstream material. To that point, Longinotto and her editors have dealt in obscurities and oddities. Then suddenly we happen upon shots from Brick Lane and My Beautiful Laundrette.

"Well I remember Laundrette being a shock when it came out," she says. "It did feel ahead of its time. Here was a mixed-race couple and a gay couple caught in the same scene. We hadn't seen that much."

Kim must have some slight personal understanding of the issues raised in Stephen Frears’s film. She is, after all, from an immigrant background herself.

“Well, I came from a sort of self-hating Italian background,” she says. “We weren’t allowed to speak Italian. But when my father had a stroke, he actually lost the use of the bit that spoke Italian. He could only speak English. But he always had this strange idea that English and German was best.”

She has, in earlier interviews, explained that she had an unhappy sort of upbringing. Her dad was not kind. Boarding school was ghastly. Before making her way to college she even endured a period of homelessness. What went on there?

“Well it was not a bad time like these poor kids in Rochdale or anything,” she says. “But I didn’t like myself for years and years. It took me years to be able to be happy. I felt that if people didn’t like me there must be something wrong with me. But, look, I wasn’t like those poor people you read about. My dad didn’t do terrible things to me. He just wasn’t very nice.”

It was her old chum Nick Broomfield, among the UK’s best-known documentarians, who persuaded her to go to film school. Unlike him, she ended up staying behind the camera when shooting her films. Was that a conscious decision?

“I never had Nick’s confidence. I may be wrong here, but I don’t think you make decisions like that. You make films the only way you can. Just as a writer writes the way only they can. You do only what you can do.”

That makes lovely, lovely sense.