Celtic music used to be out on the fringe, where true-blue folkies praised its truth and simplicity.

Then came "Riverdance," and suddenly Celtic music, blended and amplified, developed a mass following. Bands like the Chieftains, after 30 years of dedication to traditional Irish music, released crossover albums with the Rolling Stones ("The Long Black Veil") and Lyle Lovett ("Down the Old Plank Road").

"There's a really fine line" between experimentation and exploitation, says Ryan MacNeil, keyboardist for Slainte Mhath, the Canadian all-instrumental quintet that is the first of three featured bands to perform at Celtic Classic this weekend. "You have to have some respect for tradition, even if you're going to create something a little bit new."

MacNeil is the fifth of six siblings who were born at exactly the right time to ride the wave of Celtic popularity. Among them, the MacNeils account for a major part of two of contemporary Celtic music's brightest bands.

The Barra MacNeils, a foursome of older brothers Sheumas, Kyle, Stewart, and sister Lucy, are the more established group. They've toured and recorded since 1986.

"We used to play with them quite a bit," says Ryan, speaking for himself and his brother Boyd, who plays fiddle, bouzouki, octave mandolin and guitar. "But this sort of developed over the years."

What happened really is that he and Boyd were members of a pipe band from Cape Breton's Gaelic College of Celtic Arts and Crafts that traveled to the World Scottish Games in 1994. It was led by a pipe major named Bruce MacPhee. During the month-long trip, the MacNeil brothers and MacPhee became part of a ceilidh band that was featured in the pipe band's concerts. When they returned to Canada, MacPhee and Ryan MacNeil were roommates at St. Francis Xavier University in Nova Scotia.

When the Barra MacNeils came to play at the university, Sheumas asked his younger brothers if they'd like to open for them. That was the beginning of Slainte Mhath, which means "to your good health" and is pronounced "slawncha va."

"Their music is a little bit closer to the traditional side of things," says Ryan, 26, by telephone from Halifax, where Slainte Mhath played early this week for a bankers' convention. "We're a little more progressive, and more dance-oriented. They all sing; we're all-instrumental."

By "progressive," he means the band freely borrows from other kinds of music, from traditional to rock. Their style has been called "Celtic fusion."

Part of its fresh sound comes from an eclectic mix of instruments and part from the players' eclectic musical ancestry. Lisa Gallant, who plays fiddle and the Irish frame drum called the bodhran (pronounced boran), is Acadian and an accomplished step-dancer.

John MacPhee, who replaced Bruce MacPhee (to whom he's no relation) in 2001, comes from Prince Edward Island. He is a competition-level bagpiper who plays Highland bagpipes, Scottish reel pipes and Irish flute.

Drummer Brian Talbot has no background in traditional music. "He's played with swing bands, rock bands, and right now he's working as a house music DJ," says Ryan.

He says the key to their collaboration is simple: "We all listen to each other."

Celtic music seems to be related to other traditional music by little more than six degrees of separation. The MacNeil family comes from Scottish stock that emigrated to Cape Breton, where French and Irish émigrés lived side by side with the indigenous Mi'kmaq people. In their festivals and rituals, they all drew from one another. The band recently played in northern Sweden, where their music crossed the Arctic Circle without losing anything in translation.

The band's current album, and its touring repertory, ranges from Scottish, Irish and Cape Breton traditional material to contemporary pieces by Scottish, Australian and Cape Breton composers. "Myself and Boyd, we wrote a few of the tunes," Ryan says.

Their music accepts bagpipe and bouzouki, sampling and looping with equal generosity. The only common denominator, Ryan says, is that it's "very energetic. It makes you want to move, that's for sure. You just can't help tapping your feet."