Dick Gaughan/Steve Earle, Barbican, London

It's a thrilling match; Scotland's Dick Gaughan and the country rebel Steve Earle on the same bill. Both radical singers, steeped in transatlantic folk traditions that share common roots, the duo are brought together for Freedom Highway, the Barbican's festival of music and protest. With the acclaimed traditional Irish singer Karan Casey offering a more lyrical take on history and political engagement, it's an evening awash in the power of narrative song.

Gaughan, who is one of the great interpreters of the Scottish ballads, opens with a fervent and impassioned solo set. Ponytailed, and dressed workmanlike in blue jeans and shirt, Gaughan's remarkable guitar playing propels and underpins that astonishing voice, the kind of voice that could stop a train in its tracks. There can be few other singers capable of turning from aching tenderness to the high dudgeon of political rage within the space of a line, or, on occasion, even in the turn of a single word.

Conscious of traditional music's ever-present political engagement - "teenage angst is a relatively modern invention," he says at one point - Gaughan's songbook is deeply embedded in the politics of cultural emancipation, whether it is Robbie Burns or Leon Rosselson. Gaughan sings a song in Scots, once denigrated as the language of ignorance and poverty, that he first heard from the poet Hamish Henderson. It is one of the highlights of an all-too-brief eight-song set that includes Pete Seeger's anti-Vietnam song "Waist Deep in the Big Muddy" - "I'm terrified this song is more relevant now than when it was written" - and one of his own classics, "Outlaws and Dreamers", dedicated to the McCarthy-blacklisted writer Yip Harburg, the man who wrote "Over the Rainbow".

After a fine set by Karan Casey, Steve Earle strides out with his guitar and harmonica to deliver an electrifying performance. His rich, gravelly voice is redolent of mean city streets, desolate open spaces, and the yearning of the beaten-down and disenfranchised. "I was getting along fine writing chick songs," he says, picking his way through one of his ambling introductions, "but this war started and I have to start writing this political shit."

For all three performers, what's happening now, and the light the old songs shed on the present, is an overriding theme. Unlike a lot of contemporary rock music, we are not faced with a heritage industry, but with the real thing.

Earle is a singer who knows his history and his roots, and how old brutalities repeat themselves, on today's Death Row or in the racial politics of a song like "Taneytown". His song "Dixieland" weaves the Irish famine and mass immigration into the Battle of Gettysburg and the issue of slavery at the root of the American Civil War. "There is no Left in my country," he remarks before launching into "John Walker's Blues", "but it's not them. It's us. We went to sleep and let it happen. It's up to us to change it."

Tonight's airing of impassioned, personal politics, rooted in an awareness of your own history rather than what one is told, is what makes these performers stand out from the new century's culture of political apathy. The public language may well have been decimated by mass media control, but in the age of Clear Channel (the US broadcasting monolith that turned on the Dixie Chicks last year, and is looking for a foothold here) the old tongues can still prophesy and speak truth, and the message tonight is strong and clear. It is rare to find such seriousness of intent matched with this quality of performance. As the sold-out audience demonstrated, it's a combination that's impossible to resist.