When I was small my father used to tell us tales of when he was "down in Texas", complete with quick-on-the-draw finger gestures. I believed him until I was nearly 10, whereupon I realised a man who didn't like venturing beyond west Cork was unlikely to have been a cowboy. He had, of course, grown up watching the great American Westerns - Stagecoach, Rio Bravo, High Noon. He loved the romance of the lonely rebel, the freedom of those wide-open spaces, the grizzled hero doing what he's gotta do.

At exactly the same time in Mussolini's Rome, according to Christopher Frayling's new biography of Sergio Leone, the young director-to-be was seeing the same sort of films and having exactly the same reactions. Though "emergency" Ireland was a hard day's ride from fascist Italy, the American movies represented freedom and possibility in both. Sergio Leone, the son of a film director and a silent movie actress, went on to recreate the films of his youth in his own image - gunslingers complete with Catholic imagery and postPuccini soundtrack. The "spaghetti western" was born. Though the term makes the genre sound like a joke, these movies were about far more than just shifting the western to Italy or Spain. Films like The Good, The Bad and The Ugly or Once Upon A Time In The West actually redefined the American west. Many of the key traditional westerns were made by immigrants. The director of such quintessentially rose-tinted views of the West as Stagecoach and The Searchers, John Ford, was born Sean Aloysius O'Fearna. His brooding heroes and lonely homesteads are instantly recognisable to anybody familiar with the literature of the Irish west. Ford's sentimental, poetic images were of an idealised, wide-open America - the way he wanted it to be rather than the way it was.

By the 1960s, John Wayne heroism was out of date. Vietnam-era America could not believe in black-and-white heroes and bad guys. In any case, many of the real-life characters of the American west (incidentally, some of these, like Butch Cassidy, Jesse James or Billy the Kid were also of Irish extraction) were being rewritten by historians as money-grabbing renegades, not above shooting adversaries in the back. It was time for another take on the west.

Step forward Sergio Leone, a man far more interested in the immigrants who didn't make good than stories of the American Dream. Revenge or rough justice or the promise of a fistful of dollars drove his gunslinging protagonists, who immediately struck a chord. Frayling tells the story of a triple bill of Sergio Leone films he saw in San Francisco in 1968. The audience was full of Vietnam soldiers on leave or young draft dodgers. When the hip, lean, snake-eyed Clint Eastwood character in The Good, The Bad and The Ugly looks at the troops killed in the American Civil War and says "never have I seen so many men wasted so badly", the audience cheered. Here was an image of the American west that was absolutely of its time.

Frayling wrote this book in another Wild West, the Burren - he says Leone's films are more available in Co Clare than in London. Perhaps that's not surprising - when you watch A Fistful Of Dollars, there is something about that ridiculous figure that rides into town on a mule that reminds you of the gormless Christy Mahon; then he whips that hat off and you see it's the cigar chomping young Clint Eastwood, and you do get to thinking he's the Playboy of the Western World.

I doubt that Leone took in J.M. Synge on the way to the studio, however. In fact, Frayling, who is professor of cultural history at the Royal College of Art in London, reckons Leone was never much of an intellectual, or even a reader. What he had was an incredible instinct for what would work on screen. That opening scene of Once Upon a Time In The West, with three gunmen waiting for a train that's four hours late is magnificent. A windmill squeaks, water drips, a fly buzzes, but nothing actually happens for 10 minutes or more. Perhaps those Guinness commercials where the lads are all waiting in an island pub while the clock ticks endlessly were at least informed by this scene. Ritual is all to Leone.

The use of music, too, is key. Leone worked with Ennio Morricone, an old school-friend, to create a signature tune for each character. This music - what Frayling calls "post-Beachboys, post-Shadows music with a touch of Italian opera" - was played endlessly on set, so each character knew the tune and moved accordingly. The fact that Leone spoke only two words of English was no hindrance; the rotund director in wire glasses mimed out how the coolest man in the west should pull his gun: "Watch me, Clint, watch me."

This is an epic biography with each film meticulously tracked and assessed at length. Frayling freely admits to being an obsessive and jokes about tracking down every location Leone used in Spain with such a fervour that he "virtually knew the blood group of the horses". But even Frayling fell out of love with his subject in the end. Sergio Leone was a difficult man, awkward in company and even, it seems, with his family. He was overheard telling an educational bed-time story to his small children. "The first thing to learn," he said, "is never put a cent of your own money into a picture."

Though Henry Fonda was later to declare that he was the best director he'd ever worked with, Leone fell out with almost all of his stars at one stage or another. And reading this biography you can see why. He chose to shoot the only sex scene of Once Upon A Time in the West on the very first day of filming, which can't have been easy for Claudia Cardinale and Henry Fonda - they had met each other minutes before.

At least Claudia Cardinale had a decent role to play as the feisty Jill McBain; most of Sergio Leone's films avoided female characters completely. Frayling says he persuades women students to study Leone's work by telling them that "it's like 19th-century opera - it's not much fun to watch a hysterical female dying on stage and singing at the same time, but aren't the tunes great?"

Sergio Leone: Something To Do With Death by Christopher Frayling is published this week by Faber