If you think Celtic music means rustic old fiddlers and be-kilted bagpipers, you probably haven't seen what's going on in Boston these days. In concert halls and coffeehouses, at dance parties called ceilidhs (kay'-lees), and pub jams called seisiuns, the face of Celtic music is changing. Increasingly, its new face is urban, young, hip - and American.

"There's more happening now with Celtic music than I can ever remember," says Brian O'Donovan, host of A Celtic Sojourn on WGBH 89.7. WGBH expanded his Saturday afternoon show to three hours last year, another sign of the music's growing popularity.

"The number of young players coming into the scene is absolutely phenomenal," he says. "When I came to Boston from Ireland in 1980, there were two seisiuns in town. Now you can have your choice of great seisiuns every night of the week.

Spurring this, O'Donovan says, is a wave of savvy young musicians, mostly American, who have moved to the Boston area in the past few years.

"These young players bring a tremendous level of musicianship, [with] many of them educated at Berklee or the New England Conservatory," says O'Donovan. "They have a deep respect for the traditional roots of the music, but also a sense of open-mindedness and adventure.

"They all have a desire to play just for fun. They're making a living at it, but they really understand that this is, at its heart, a social music."

Hip, urban, and traditional

The new players bring an urbane, street-smart vibe to the old music that is attracting legions of new fans. Club Passim in Harvard Square is known as a hive for edgy modern songwriters. Never in its long history has it hosted as many Celtic shows as it does now.

"I think these young Celtic artists are really creating a new energy around [the music], revitalizing it while staying true to the traditional essence," says Passim manger Matt Smith. "It's been so much fun seeing our regular songwriter crowd get blown away by Celtic music."

Local fiddle trio Halali sold out its recent Passim show so quickly that a second show was added, and then sold out. So when Halali member Laura Cortese wanted to bring her own band in, Smith quickly agreed. One night in late February, the club was packed with just the sort of young, urban crowd you'd expect at Passim. And they were listening to Celtic music.

It was striking how much Cortese, with her black, red-streaked hair cut in a chic bob, resembled her audience - except for the fiddle.

Backed by cello and guitar, she began with a tender air that she stoked to pulse-pounding quickness. Then she kicked it up even more, and was met with the crowd's excited whoops.

Without surrendering the melodic grace that makes Celtic music so captivating, she coaxed ambient, techno-smart brush strokes from her fiddle as she sang an ancient lament that, in her voice, sounded more defiant than weepy. The music was deeply traditional, but the presentation was modern.

Cortese, who grew up in San Francisco, exemplifies the new generation of Americans who are shaking up the old music these days. Though Italian-American by birth, her fiddle style is Scottish, rooted in the time she spent at fiddle camps run by Scottish fiddle phenom Alasdair Fraser. That's also where she met the other Halali fiddlers, Hanneke Cassel and Lissa Schneckenburger.

All three moved here to add some classical theory and music-business education to their fiddle savvy: Cortese and Washington state-born Cassel graduated from Berklee, Maine native Schneckenburger from the Conservatory. But they learned Celtic music in traditional, social ways, from older fiddlers and at fiddle camps.

"Lots of people my age are thinking if there's one city where I can live and be able to play tons of this music, Boston is the place," Cortese says. "Irish and Scottish people have been living here since Boston was Boston, and there's also a huge Cape Breton community. So there's always been a general culture of people supporting this music."

The Celtic music community

If there is an epicenter for these new players, it is the Burren pub in Somerville's Davis Square. Stars from all over the Celtic realm, including the Chieftains, Solas, Eileen Ivers, Altan, and Old Blind Dogs, have been been seen there after concerts, sharing a pint and a tune at the nightly front-room seisiuns. Maybe that's because Irish musicians Tommy McCarthy and Louise Costello own the place.

"Back in the '70s, this was more of an ethnic music," McCarthy says. "If you were at an Irish seisiun, your surname was probably O'Sullivan or Murphy. Now you look at the seisiuns in the front bar, and there's all kinds of nationalities enjoying this music and playing it."

There are nightly seisiuns at the Burren where players gather informally to take up instrumental tunes together, but Sunday's is the most open welcoming to newcomers who want to sit in. Sunday seisiun host Shannon Heaton, a flutist who moved here from Chicago with her guitarist-husband, Matt, warns that seisiuns are not exactly open stages.

"It's most fun when you have a bunch of people around the same level, because you can push each other," she says. "As a host, you pay attention to what's going on, listen to everyone who's playing, and aim for the highest common denominator. You try to set a good tone, a good speed for everybody to play together."

She advises newcomers to any seisiun to hang around awhile before asking to play. Listen to the level of play and decide if you can comfortably keep up. It's a bit like surfing: The wave won't slow down for you, it'll just let you fall off. Watch the social interaction as well, because each seisiun has its own etiquette. Chat with people at the bar and with musicians on break, before you ask if you can sit in.

So you want to play?

There are two local seisiuns designed for newcomers. Hosted by local fiddle legend Larry Reynolds, they happen Monday nights at the Green Briar in Brighton, and Tuesdays at the Skellig, the Burren's sister pub in Waltham.

The Green Briar even has two special "slow seisiuns" for beginners before the main seisiun begins at 9 p.m. The idea was hatched four years ago by Bill Firla, who took up fiddling after retiring from IBM in 1996. As cohost Nancy Kleiman explains, "Some people would sit at a seisiun for five years before they felt comfortable enough to play. Bill thought it would be nice to have a seisiun that was slow enough for new players to join in right away."

That tempo sped up over the years to about half the speed of an average seisiun, so a few months ago, a seisiun strictly for beginners began in a Green Briar banquet room.

At the intermediate seisiun, 14-year-old Brookline student Amanda Cavanaugh sat next to 17-year-old Japanese foreign exchange student Taku Murakami. Both are promising fiddlers, often earning approving grins from 84-year-old spoons player John Hennessy, a retired bartender from Cambridge.

Joshua Lieberman of Newton fiddled along with his 9-year-old daughter Michal. He has been a hobbyist fiddler since college, but Michal didn't catch the Celtic bug until last summer, when the family vacationed in Cape Breton, a part of Nova Scotia that is steeped in Scottish-based music. She also studies classical violin and relishes the carefree way Celtic music is played.

"It's not like there's one right way to hold your violin or put your fingers down, so I can play it on my stomach if I feel like it," she says with a giggle. "You can play it however it sounds best to you and just have fun with it."

"I go to her classical concerts," says her father, "but there aren't that many things families can do together these days. That really impressed us about the music we saw in Cape Breton; the tradition is that everyone joins in. It gave us the dea that this was something she and I could do together, even though she's already better at it than I am."

At the end of the day, as the Irish would say, that's the biggest and best reason Celtic music is on the rise these days. The social heart of the music attracts hobbyists and stars alike, for whom spreading the joy is part of the fun.

All the Halali fiddlers, as well as the Heatons, work as teachers and cultural activists, organizing events such as January's Boston Celtic Music Festival. As Cortese puts it, "It's all about entertaining each other; not just one person entertaining everybody else."

"That's certainly what drew me to this type of music, that it's a community-based form," she adds. "When kids teased me about playing this dorky fiddle music, it made me think, `Man, they've got to see this community.' Instead of feeling bad that they thought I was dorky, I felt like, if they only knew what they were missing. I'd say, `No, this is cool; I know it's cool. Here, let me show you how cool it is.' "