“Others said ‘Good luck, but we don’t want to be depicted’, so we promised them that they’d not be mentioned. We would focus only on members of the Support Group, who felt that the story had been swept under the carpet for too long by governments and forgotten in the press, too.” They also sensed an opportunity to attract valuable publicity to the civil action they had launched against five Real IRA suspects, and, crucially, they had seen Bloody Sunday. “It convinced them that they wanted this film to be made,” says Dyke. “They said ‘Don’t shy away from showing the horror of the blast’, but we made it clear that we wouldn’t show their loved one dead without their consent. Some of them took a while to grasp that they were going to be played by an actor, because they were so used to documentary crews filming them.”They wanted us to include the small things, such as a fund-raising football match or the vandalising of their memorial garden, but we told them we had to do the big picture – police and government.” Ed Guiney, Omagh’s Dublin-based producer, says: “The Omagh families needed to be happy with the way that we’ve told the story, but Omagh is a piece of drama, so certain details and events have been created. But nothing’s been substantially altered.”With the Support Group on side, Greengrass and Guy Hibbert, writer of the BAFTA-winning drama No Child of Mine (1997), initially worked on a script telling the story of the bombing from the viewpoints of three of the Omagh bereaved: an Ulster Protestant, an Ulster Catholic and an Englishman. “We decided that the script didn’t seem to work in that form,” recalls Dyke.

The 100 minutes available were not enough to do justice to all three stories.Instead, the focus shifted onto Gallagher, who, on 15 August, 1998, was working on a car with Aiden, then 21, when his son left to go shopping in town with a friend, and became the second victim of terrorism in the Gallagher family. Michael’s younger brother, a former Ulster Defence Regiment soldier, had been shot dead by the IRA in 1984, a victim of the Provisionals’ policy of murdering Catholics who joined the army (Omagh skilfully withholds audience knowledge of the earlier death until its mid-point, when Gallagher reveals it during a fruitless meeting with Gerry Adams).As the script neared completion, Greengrass reluctantly decided that he would not direct Omagh as originally planned, tempted away by $100m-plus action sequel The Bourne Supremacy. “After a three-year battle,” he told reporters, “we are on the way to trial. There will be a message from the film that there’s hope in the despair.” On 23 April, he faced the cameras yet again, this time outside the High Court in Belfast after a preliminary hearing of the families’ civil action against the Real IRA, at which the judge said that the full case would open on 17 January 2005. We promised our loved ones and supporters that we would not falter in pursuing this matter to the end.

Now the end is clearly in our sights.” He’d said something similar, though more personal, perched on a worn sofa in a St Brendan’s corridor. “Gerard has a direct physical connection to the town and brings a deeper passion to it than someone unconnected would, and that will have a bearing on his performance.”In January, Gallagher anticipated that Omagh would be “a very, very difficult film for us to watch, and some will choose not to That’ll be their coping mechanism. This is a story about a great tragedy and I like to think that in the midst of evil, ordinary people who wouldn’t normally come together have done so, in the search for truth and justice. He rejected wearing a wig and glasses to create even an approximate resemblance, “because I can portray the psychological life of the character without that external stuff.” He was born in Omagh, where his father ran a bicycle shop and Gallagher believes the actor’s roots are an obvious plus. “But playing Michael is a graver responsibility to me because of the weight of his and his family’s calamity and suffering.” Actor and character are physically very different: Gallagher a bear-like figure with glasses and thick white hair; McSorley bald and perhaps six inches shorter. Running through the script almost like a refrain is the families’ belief that politicians in Belfast, Dublin and Westminster did not want the hunt for the bombers to succeed because an Omagh bomb trial would disrupt their post-Good Friday plans.

As Michael Gallagher tells a TV reporter in one scene: “Everyone wants us to walk away, go quietly. So that they can go on with their peace process.”McSorley’s low-key performance – he’s at the centre of almost every post-bomb scene – carries Omagh. One of Ireland’s best and busiest screen actors, he’s now completed a hat-trick of real-life roles: first in Bloody Sunday as Chief Supt Frank Lagan, the only Catholic in the RUC in 1972, then as vicious Dublin crime baron John Gilligan, terrorising Cate Blanchett’s investigative journalist in Veronica Guerin; and now Gallagher, whom we see finding great inner strength even as his obsessive campaigning threatens his relationship with his family.”I’ve treated all three men the same way from a technical point of view,” McSorley says. Lorcan Cranitch’s cameo as Sir Ronnie Flanagan, insufferably smug and evasive as he meets the Support Group, does nothing for the former RUC Chief Constable’s reputation. “It’s a question of how close you can go to reality while also moving away from it, so that when they see Omagh nobody will scream about inaccuracy or about our having been over-graphic.” Omagh’s recreation of the post-bomb chaos has blood-stained, bewildered shoppers crying for help, one man who’s lost a leg and another carrying a lifeless child.These shots provide the most shocking moments in a film whose admirable restraint is unlikely to stop it from reawakening the controversy over the Omagh investigation. They supported us because they said it was a story worth telling.” In Navan, production designer David Wilson’s task was to dress and then distress the shop fronts to match Omagh’s, and replicate what he calls the “iconic” newspaper photographs of the bomb damage, with emergency personnel surrounded by debris in the foreground and the Omagh courthouse in the background, at the top of the Market Street slope.”When you’re designing a factually-based drama there’s a lot more pressure on you than with fiction, but you still have to use your imagination,” says Wilson, who recreated the interior of the Maze prison for Some Mother’s Son (1996), about the IRA hunger strikes.