Think back to 2001. Martin Scorsese is about to cast Brendan Gleeson opposite Daniel Day-Lewis in Gangs of New York but Steven Spielberg is lining him up for Artificial Intelligence: AI, which is due to go into production at the same time. The two directors get together and juggle their shooting schedules to allow the Dubliner to star in both films.

Amid all this, Tomm Moore and Paul Young, a couple of unknown young animators just out of Ballyfermot College, approach Gleeson to see if he'll become involved in a cartoon feature they're hoping to make through a company they formed while working on a Fás scheme.

"They told me what they were doing and I said yeah," Gleeson recalls.

Or maybe that was a little later, just after he'd played a murderous fiddler in Anthony Minghella's Cold Mountain and was preparing for his role as Menelaus in Troy, where he had a fight to the death with Brad Pitt.

"I can't remember for sure how it all began, it's all a blur those years," Gleeson says. "But every so often they'd send me things and let me know how they were progressing. I did the voiceover for a trailer they used as a calling card to raise finance. They'd a script they sent me and they came out to the house at one stage and we had a chat. You could see they were fellas who were doers rather than talkers. You just knew they were the real thing."

Moore and Young's idea was to create a fictional adventure fable about the origins of the Book of Kells, the fabulously illuminated Latin gospel now regarded as Ireland's greatest national treasure. A young orphan Brendan is growing up in the abbey of Kells, around which the stern abbot – voiced by Gleeson – has built an enormous wall to hold out Viking hordes on a rampage of pillaging and slaughter. All the resources of the community have been diverted from their task of inscribing mankind's accumulated human knowledge in meticulously crafted illustrated books to strengthening the fortifications.

"Although it's the distant past it's very timely," says Gleeson. "You can take so many metaphors from it for now in terms of threats to come and what fear can do to people."

One of the greatest of all illustrators, Brother Aidan – with his cat Pangur Bán – finds refuge in the abbey, bringing with him from Iona a book rumoured to be the most beautiful ever transcribed. He becomes a mentor to Brendan, passing on to him the secrets of his art. Defying the abbot, Brendan slips out beyond the walls into a wild forest where mythical creatures lurk in search of the oak berries Aidan needs for his inks, and somehow survives with the playful help of a mischievous girl sprite Aisling.

"My character changed over the years," says Gleeson. "The whole script changed. The title was originally Rebel, now it's The Secret of Kells. It's over three years since I recorded my part so I didn't know what to expect. I certainly wasn't prepared for the extraordinary originality and visual beauty that filled the screen last Sunday at the Dublin Film Festival. Some of the frames are so beautiful you want to run away with them."

The Secret of Kells is at once a feast for the eyes and enchanting entertainment. Defying the dominance of 3-D computer-generated imagery, Tomm Moore has found a way of marrying traditional hand-drawn two-dimensional animation to the breathtaking intricacies of medieval art and calligraphy to bring alive a magical fantasy of extraordinary beauty and charm. He uses split-screen techniques to mirror the spatial spread of medieval triptychs and plays on their parallel with graphic novels. Paul Klee famously described drawing as "taking a line for a walk". With Moore it morphs into illuminated manuscripts that jump off the page.

French critics are already in ecstasy. "The visual context is so splendid that it keeps the viewer in a daydreaming state from start to finish," enthused Le Monde.

The film's aesthetic brilliance never takes away from its prime purpose to thrill young audiences, who love to be frightened as long as they know it's all pretend. "It even scared me," says Gleeson. "I've been fascinated by the Vikings since I was in short trousers, but this was the first time I was ever afraid of them. There's real evil in their presence."

The Secret of Kells confirms the growing status of Irish animation as a force in world cinema. Breakthroughs in digital technology and the internet are making it possible to create high-quality animation without the vast resources of major studios like Pixar and Disney.

Moore and Young and their mushrooming team of artists at Cartoon Saloon in Kilkenny are cleverly taking advantage of this opening to allow Ireland to compete at the highest level.

"We started up in 1999 when the Don Bluth studio was closing down," says Moore. "By doing stuff on the internet we were kind of connected to other animators around the world. Our whole business has been based on the fact that we can do everything from Kilkenny. We don't have to be in London or LA."

The trailer for The Secret of Kells helped the team get advertising jobs and led to their award-winning flash-animated television series Skunk Fu!, which was taken up by BBC and TG4 and became a hit in the US and Australia. "At one stage we were operating on three floors in Kilkenny, with Skunk Fu! on the top floor, Kells underneath and other work on the ground flour," says Moore.

Cartoon Saloon is a nursery for young animators. Local schoolboy David O'Reilly, who was there on work experience as a 15-year old, has gone on to win the Golden Bear for shorts at the Berlin Film Festival with Please Say Something, the first time since John Lasseter that it's been won by an animator. "He was always hanging around the studio and we were impressed by how talented he was. Early drawings for Brendan were based on him at that age."

Gleeson sees Cartoon Saloon's success as a welcome fillip at a time of national pessimism. "I'll have to guard myself against preaching, but you want to get people hopeful again, to regain a sense of themselves and of what's possible. These guys have done it over the past six or seven years. They've actually produced something of extraordinary beauty. At one stage they had 75 animators from all over the world in Kilkenny working on this film. Now that's proper entrepreneurship. It's about creating things, not about bubbles."

Gleeson should know. Back in the late 1980s he first came to prominence along with fellow schoolteachers Paul Mercier and Roddy Doyle – who on Friday received the prestigious annual PEN award for Literature – writing plays for Passion Machine, an innovative theatre group that opened up new audiences and found drama in the everyday lives of ordinary people.

Gleeson, who received a Golden Globe nomination for his performance as a hitman-with-a-conscience in Martin McDonagh's black comedy thriller In Bruges, has been in Morocco filming Green Zone. It's a political thriller set in Iraq during the US occupation. It reunites Bourne Identity star Matt Damon with Paul Greengrass, a director who maintains film shouldn't be "disenfranchised from the national conversation. It is never too soon for cinema to engage with events that shape our lives."

Gleeson reputedly plays the CIA's Baghdad bureau chief, but he's reluctant to confirm this. "We're not supposed to talk about what anything's supposed to be. And there's no point talking about it because each time I went back it kept changing. We'd finished the film but then did a lot of re-shoots before Christmas. The script really has nothing to do with anything for Paul. He keeps throwing it away. He has this extraordinary sense of the veracity of what he wants to get at."

It was something of a culture shock going straight to Green Zone from In Bruges. "Martin McDonagh is the opposite to Paul. We had weeks of rehearsal and we sat down in a room and we talked through the minutiae, every word and comma. He's such a keen writer you don't want to change things."

Gleeson will next be seen as Winston Churchill in Thaddeus O'Sullivan's biopic Unto The Storm. Could the global economy do with a touch of the Churchill spirit right now? Gleeson isn't so sure. "He got turfed out at the end of the war because he was 'steady as she goes', trying to go back to the old ways."

Having just completed Ian Fitzgibbon's comedy thriller Perrier's Bounty – "I've decided I need to revert to my psycho side" – Gleeson is now preparing to put back on his black eye-patch as Mad Eye Moody for the seventh and final Harry Potter film, Harry Potter and the Deathly Hallows. All he knows about it is that he'll die. "I've absolutely no idea how. I haven't seen the script yet. I think there'll be a certain amount of skydiving. Or maybe it will be a whimper. It's only alluded to in the book."

Meanwhile, Tomm Moore has already started on Cartoon Saloon's next film The Song of the Sea, a story about fairies trying to open up a portal to return to their world because they can't exist anymore in our world. "It's inspired by the memory of a big storm we had in 1987, when I was 10 years old," he says. And another Cartoon Saloon animator, Nora Twomey, is developing a live-action feature based on Bluebeard, a man who kills his wife and locks the body in a room to hide it from his new wife. It'll be done on green screen, somewhat in the manner of Sin City but looking like paintings by 19th century Romantic artist Casper David Friedrich.

"Hopefully we can now go to a new level," Moore says.

The Secret of Kells opens on Friday