It all started at the National Folk Festival.

Ten years ago, Joe Wilson, the director of the National Council for the Traditional Arts, asked Seamus Egan to gather some friends together to play traditional Irish music in Lowell, Mass. So Egan, who is hailed as a multi-instrumental virtuoso, recruited a few all-stars in the genre: fiddler Winifred Horan, concertina and accordion player John Williams, guitarist John Doyle and vocalist Karan Casey.

It was impromptu – they didn’t even rehearse – but as they played together onstage, things just clicked. The musicians had a ball, and the audience enjoyed it even more.

“We thought, ‘Maybe this is something we should pursue a little further,'” Egan said by phone from Philadelphia, where he and Horan live.

The group became Solas, Gaelic for “light,” and released its first album in 1996 to popular and critical acclaim. The Boston Herald declared Solas “not merely America’s best Irish traditional band but maybe the world’s.”

Though the band’s lineup has changed several times in the years that have passed – Solas now includes Egan and Horan, as well as Mick McAuley on accordion, vocalist Deirdre Scanlan, and Eamon McElholm, a keyboardist and backup vocalist – it hasn’t distilled their sound.

“I think anytime you have any kind of change in a band it’s a challenge, but we’ve been fortunate,” Egan said in a soft brogue – though he was born in Pennsylvania, the family moved to Ireland when he was 4. “The whole idea of Solas all along is everybody in the band being themselves and bringing what they have into the collective.”

Most bring a pedigree. Egan has released three solo albums, he collaborated with Sarah McLachlan on “I Will Remember You,” he wrote the soundtrack to the movie “The Brothers McMullen,” and his music also can be heard on the “Dead Man Walking” soundtrack. Horan, a New York native, was a fiddler for Cherish the Ladies. In addition to two solo albums, she also has recorded with the likes of Patty Larkin and Eileen Ivers. She’s also a nine-time Irish step-dancing champion, and she’ll treat visitors at this year’s National Folk Festival to a step or two.

Scanlan joined the band in 1999, shortly after her solo debut garnered widespread praise in her native Ireland. Scanlan’s fellow countryman McElholm is Solas’ newest member. While studying in Manchester, England, McElholm won the Performing Rights Society-John Lennon Songwriters Award, and he later worked with the popular Irish band Stockton’s Wing. McAuley is regarded as one of Ireland’s finest accordionists, and he recently released his first solo album.

As the band’s membership has evolved, so has its music.

“We’ve gotten a little more advanced in our choice of material, particularly songs,” Egan said. Solas’ last album, “The Edge of Silence,” included covers of songs by Bob Dylan, Nick Drake and Tom Waits. “We’re expanding instrumentation on our albums and availing ourselves to some of the technology, as well.”

Though the band’s earliest music fell well within the realm of traditional Irish music, they were always open to new ideas. It’s a fine line to walk, however. When people go out to hear Irish music, they expect to hear a jig. When they don’t, some go home unhappy.

The same can be said of Solas’ recent releases, which have been met with mixed reviews. Some critics were left scratching their heads, while others extolled the band’s fearlessness. A Billboard reviewer wrote of “Another Day,” “Solas manages to be contemporary without being overtly modern.”

“Even from the beginning, we never said, ‘OK, this is what we’re going to be,'” Egan said. “We don’t want to get complacent. I think it’s important, even if people get annoyed, it’s important to keep pushing. It doesn’t always make things easiest, but it seems to work for us.”

It’s been working since that first folk festival, nearly a decade ago. In retrospect, it was the perfect place for a band like Solas, which embraces diversity and musical experimentation, to get its start.

“One of the things I think is just fantastic is it brings together such a wide range of artists, who even at the most adventurous festivals, never get together,” Egan said. “The wide range of different types of music and art is just extraordinary.”