`She'll stick to him," James Joyce's father observed when he first heard Nora Barnacle's name. Pat Murphy knows all about it. For 10 years her life has been dominated by the young Galwaywoman who ran away from Ireland with Joyce, and she pursued her goal of bringing Nora's story to the screen through multiple setbacks, disappointments, changes of plan. It became a personal crusade, now realised in Nora, which opens the Dublin Film Festival. Directed by Murphy and co-scripted with Gerard Stembridge, it stars Susan Lynch as Nora and Ewan McGregor (also a co-producer) as Joyce.

"I needed to do it," Murphy says. "Even when I felt discouraged, I found it impossible to walk away from it. These people - Nora and Joyce - refused to go away."

The film concentrates on eight years in their lives, from their first encounter on Grafton Street in 1904 to their departure into exile a few months later and the early, stormy years in Trieste ("la bella nostra Trieste"), where their two children were born and Joyce wrote Dubliners and Portrait of the Artist.

The focus is firmly on Nora, superbly portrayed by Susan Lynch as a courageous, spirited woman who struggles to come to terms with the demanding, volatile and occasionally cruel man with whom she has thrown in her lot. The film observes their differences - she instinctive, spontaneous, uneducated, funny, he intellectual, complex, obsessive, needy. It also shows what they shared: a youthful fecklessness, sensuality and a love of music and fine clothes - especially hats.

It is a perceptive, sometimes moving portrayal of a relationship between two people who inhabit very different worlds but are bound by a passionate sexual connection.

"It's important that people who don't know about Joyce can watch it and experience it as a great love story," Murphy says. "I didn't take the conventional, biopic approach. I focus on their early years together, which were very important. In those years the main themes of Joyce's writing are worked out, and I wanted to show how important Nora was to his work."

Joyce drew freely on Nora's personality and speech in his work - most famously for Molly Bloom's soliloquy in Ulysses - and the film convincingly portrays this appropriation as one of the causes of conflict and misunderstanding between the couple.

When Nora comes across the manuscript of the short story The Dead, based on the tragic death of her young suitor in Galway, she accuses Joyce of twisting her life in his work. He is aggrieved that she doesn't realise that in writing about her he celebrates his love for her. At times, however, Joyce appears to be manipulating events, almost setting up situations - such as pushing her into having an affair with another man - in order to write about them.

"I'm exploring a certain part of their relationship," Murphy says. "Jealousy and betrayal were recurring themes of Joyce's work, and he wrote about real situations. Nora wants to have a simple relationship, but he complicates things and she becomes intrigued by the situation."

Joyce also encourages her to write sexually explicit letters to him while he is on a return visit to Dublin, and these keep their mutual desire alive. Susan Lynch sensitively conveys Nora's physicality and sexual openness.

The actual correspondence between them reveals Joyce's fascination with defecation and excrement, which the film does not portray, although we do see Nora drawn into his obsessions. "This was quite a dark period," Murphy says, "and she eventually draws him out of it through her humour."

The film conveys the couple's enjoyment of Italy (shot in golden light by Jean Francois Robin), free from the constraints of Ireland. Although the emphasis on their extravagance, flamboyance and carelessness is deliberate, the succession of apartments they rent and Nora's gorgeous hats, dresses and lingerie nevertheless seem a bit too opulent for their straitened circumstances in these years. The production design (by Alan Macdonald), costumes (Consolata Boyle) and cinematography combine to create a period glossiness, tending towards the picturesque.

"In the film, they do appear richer than they really were," Murphy says. "But you must remember that for people who don't have an investment in bricks and mortar, their wealth is on their backs."

Murphy's previous films were reclamations of women's experience, exploring, in Maeve (1981), the intersection of feminism and republicanism in Northern Ireland through one woman's story, and, in Anne Devlin (1983), the hidden history of women - in this case, Robert Emmet's maid - caught up in great political events. Her fascination with Nora Barnacle stems from the same impulse.

Like many people interested in Joyce, Murphy had read Richard Ellmann's biography. "Nora really doesn't figure in it and I made the same mistake as everyone else. I accepted the received opinion that she wasn't Joyce's intellectual match. It was Brenda Maddox's biography of Nora (published in 1988) that really put her into the picture." "When you read about her first, you feel that she was victimised, but I am now convinced that she was Joyce's equal. She was extraordinary, heroic. I began to feel inadequate to show what she was really like, to do justice to her."

To help her bring Nora to life, Murphy asked Gerard Stembridge to join forces with her on the screenplay. "He brought clarity and he challenged ideas and structures I had. It was very good for me to work with someone else. Up to then I had been an auteurist film-maker, working on very personal projects. I wanted to change."

She also changed the way she worked with actors, allowing scenes to develop organically in rehearsal, rather than working every detail out in advance. "I'd never worked with actors in that way before - it was terrifying and extraordinary." She emphasises the commitment of the cast and crew throughout. "It took so long to do and was so heartbreaking to get together that, if anyone had withdrawn their support or faith, it would have fallen apart.

"I'm pleased with what we've done, and I'm relieved it's over. I feel as if I've been invisible, as if I haven't had a voice for 10 years. I want to move on, try new things."

Perhaps, but she still talks about Nora Barnacle in the present tense.

Nora will be screened on Thursday, March 6th, at UGC 5 at 7.30 p.m.