While the sun blazes outside and the Da Vinci Code crowd clears out of town, Michael Dwyer, Film Correspondent, gets down to some serious viewing at the 59th Cannes Film Festival

The sun has been blazing down on Cannes all week. Perfect weather for sitting in dark cinemas every day watching movies. And the Cote d'Azur is bursting at the seams. Tuesday's front-page headline on daily newspaper Nice-Matin proudly declared that 130,000 festivaliers will be in town for the 59th Festival de Cannes.

You will have heard of some of them: actors Brad Pitt, Cate Blanchett, Bruce Willis, Gael Garcia Bernal, Sarah Michelle Gellar, Ethan Hawke, Kirsten Dunst, Cillian Murphy and Halle Berry, and a slew of directors, including Pedro Almodovar, Nanni Moretti, Ken Loach, Richard Linklater, Sofia Coppola and William Friedkin. And then there's Al Gore, the former US vice-president, who arrives here today to promote his global-warming documentary, An Inconvenient Truth.

As ever, the more high-profile folk will spend minimal time at Cannes, flying in to introduce and plug their new movies, and then heading back on the motorway to Nice Airport. Having opened the festival on a wave of hoopla on Wednesday night, Tom Hanks, Ron Howard and the rest of the Da Vinci Code team are probably packing their bags, or more likely having them packed, as you read this. Au revoir, guys.

There are, however, nine dedicated film people who will be here all the way to the closing ceremony on Sunday week and who will see every one of the 20 films in competition for the prestigious Palme d'Or. Mesdames et messieurs, put your hands together for the 2006 Cannes jury.

In keeping with longstanding festival tradition, they're an electic bunch. Representing the thespian profession are Helena Bonham Carter, Samuel L Jackson, Zhang Ziyi, Tim Roth and Monica Belluci. Joining them are directors from France (Patrice Leconte), Argentina (Lucrecia Martel), Palestine (Elia Suleiman) and China (Wong Kar-wai).

Wong is president of the jury, so the eight others can expect to have their daily deliberations conducted in smoke-filled rooms. When I interviewed Wong last year, he chain-smoked his way through our time together. Even the photograph of Wong in the Cannes brochure shows him gesticulating with a cigarette between his fingers.

Then again, just about every restaurant and bar in Cannes is smoke-filled, and the alternative is to eat and drink alfresco, passively inhaling the fumes of the incessant traffic.

It will be interesting to see how the diverse talents on the jury, coming from such different generations and cultural backgrounds, find consensus on a competition selection with such a wide range of themes, genres and styles.

Flying the Irish flag in the Cannes competition is Ken Loach's The Wind That Shakes the Barley, although in line with the policy of determining a movie's nationality by its sources of finance, Loach's film, shot in counties Cork and Kerry last summer with an almost entirely Irish cast, is listed as a British-Irish-German-Italian co-production.

I suspect that Loach will have more than a few ardent admirers on this festival jury and that they could well give him the Palme d'Or that has eluded him in the past. He has had seven films in competition at Cannes without ever receiving the most coveted award, although some of those earlier films (My Name is Joe, Raining Stones, Sweet Sixteen) collected other prizes at the festival.

Not that Loach is likely to be unduly worried about adding to his already substantial trophy collection, or that he's likely to lose any sleep between now and Sunday week, when the winners will get early-morning phone calls advising them to jump on the next plane back to Cannes for the prize-giving.

Throughout his remarkable career, Loach, who turns 70 next month, has been one of the most egalitarian, obstinately uncompromisingly and socially and politically concerned directors working in world cinema. One imagines that his greatest reward is actually getting his films made - usually achieving miracles on small budgets - and addressing the issues he wants to explore with characteristically fervent passion and a virtuoso film-making style.

Given its world premiere at Cannes, The Wind That Shakes the Barley marks Loach's sixth collaboration with the Scottish lawyer-turned-screenwriter, Paul Laverty. It is, unsurprisingly, politically loaded. Having tackled the Spanish Civil War in Land and Freedom, Loach and Laverty now turn their attention to the turbulence of post-Treaty Ireland in the early 1920s.

The experiences of a rural Cork guerrilla force of flying columns serve as a microcosm and as the dramatic conduit in a film where the recurring, twisting theme of taking sides is evident from the innocuous first scene at a hurling match. There's no doubt which side Loach takes.

From the first, early appearance of the Black and Tans, they are depicted as callous, belligerent oppressors, and there is, perhaps, one scene too many to emphasise their sadism. But the film proves more complex than that on just about every level, and a later scene observes them from a socialist perspective as servants of their political masters, as men hardened from years of being up to their necks in muck and vomit in the trenches of the Somme.

It's more complicated on the Irish side, and even more so when that side is splintered. Clearly intent on avoiding a debate about specific historical accuracy, Laverty's screenplay features fictional characters, although there is a glimpse of Michael Collins in a cinema newsreel reporting the signing of Treaty and the establishment of the Free State.

The film's preoccupation is with the background to those events, the confusion of their consequences and, implicitly, their reverberations over the next 80 years of Irish history. The dramatic prism is the relationship between two brothers from a well-to-do family. Teddy (Padraic Delaney) is committed to the republican cause, while Damien (Cillian Murphy) is a young doctor about to emigrate and work in London when he becomes politicised by events.

In the movie's most harrowing sequences, one brother is subjected to horrific torture (in which his fingernails are forcibly removed) and the other takes on the duty of executing a young friend who has been naive enough to become an informer. It is the execution that is the more emotionally wrenching scene, and the one that prefigures the hard consequences of conflict to follow, as violence begets violence begets violence.

In the pivotal role of Damien, Murphy is masterfully subtle and expressive, even in the contrived romantic sub-plot, which serves a purpose but feels dispensable. After so many remarkable performances, from Disco Pigs to Breakfast on Pluto, Murphy, who will be 30 this month, is on track towards becoming the most interesting and versatile actor Ireland has ever produced.

As his brother, Delaney is inspired casting, a thoroughly assured and impressive newcomer to leading screen roles, and it is a pleasure to see Liam Cunningham back in a substantial cinema role as a Dublin trade unionist whose avid socialism was inspired by seeing James Connolly "set the place alight" at the 1913 Lockout.

The film's production values belie its low budget. Eimer Ni Mhaoldomhnaigh's period costumes are as handsome as they are impeccably detailed, and the film is shot with the dexterity we have come to associate with Loach's gifted lighting cameraman, Barry Ackroyd.

Ackroyd also happens to be the cinematographer on another compelling factually inspired film showing at Cannes, United 93, directed by Paul Greengrass, who, like Loach, is English and worked extensively in British television before making his mark in movies.

Greengrass belatedly received mainstream recognition with Bloody Sunday four years ago and then demonstrated his dynamism in the action movie genre with The Bourne Supremacy. He imagines the unimaginable in United 93, which takes its title from the number of the plane that was hijacked on September 11th, 2001, and did not claim its target.

The available evidence - from the lovers, relatives and friends of passengers who contacted them in-flight from their mobiles - is that a plan for the plane to be the fourth weapon of mass destruction on that fatal day was thwarted when the passengers fought back.

As the film notes, there was another crucial factor at play, in that the delayed take-off of this United Airlines flight allowed time for its passengers to be alerted to the attacks on the World Trade Centre. Greengrass treats the story with sensitivity through the form of a dramatised documentary that mixes unknown actors in an impressionistic collage of passengers and hijackers, along with real-life air traffic control staff as themselves.

His film is one of those rare achievements - another is All the President's Men - which takes material that is familiar and injects it with a dramatic urgency that is riveting, even though we know the ending from the beginning and are aware of so much that is said to have happened in between.

A succinct expository prelude shows the four hijackers preparing for their mission, at prayer and shaving their bodies for the paradise they believe beckons. A telling later scene intercuts passengers and hijackers praying in panic to their different gods. Then, in what feels like a cruelly ominous scene, the camera prowls inside the empty plane before the crew come aboard.

When the plane takes off, on its scheduled journey from Newark to San Francisco, the movie shifts into real time. Down on the ground at air traffic control, when the first plane hits the World Trade Centre, we see it only as a blip on a radar screen, but we know what's coming. When the second plane strikes, the effect is devastating, no matter how many times we have seen the original footage.

As at least some of us felt when we watched that footage over and over again five years ago, the film places us in the position of voyeurs, but its approach eschews sensationalism and exploitation. Even the key line spoken by one passenger, "let's roll" (later taken by Neil Young for a heartfelt song on the tragedy), is muttered rather than delivered with the dramatic panache of a conventional Hollywood hero.

When the plane crash-lands in Shanksville, Pennsylvania, as we know it will, the final five minutes of the film are terrifying, unbearable and stunning. United 93 is showing out of competition in the official selection at Cannes, which is appropriate given how raw the emotional and physical wounds of 9/11 remain five years on.

• Michael Dwyer reports again from Cannes on Tuesday.