In a few months, Linn Barnes and Allison Hampton will be among the busiest musicians in Washington. Their "Celtic Christmas" program, scheduled for seven performances in Dumbarton United Methodist Church in Georgetown, has become one of the best-loved annual events of the local holiday season--almost as popular as Handel's "Messiah."

Late August is a little bit quieter, though. It was quiet enough to allow them to give two free performances last week in the Kennedy Center's popular and groundbreaking "Millennium Stage" concert series, which is available free in person or on the Internet.

Actually, there are two Millennium Stages, one at each end of the Kennedy Center's long Grand Foyer. Last Friday, Barnes and Hampton played on the stage next to the Eisenhower Theater.

All the seats were filled--with folding chairs, because that space has to be available later for Eisenhower Theater patrons--and a substantial overflow audience sat on the comfortably carpeted steps leading up into the theater. The audience heard a free concert that would have been a bargain at the Kennedy Center's usual prices.

It's too early for Christmas music (except for the annual "Christmas in August" program sponsored by WETA-FM in memory of the late Bill Cerri), but Celtic music is always in season. Barnes and Hampton, just returned from a visit to Ireland, had a good supply of Celtic music.

The material was mostly Irish, mostly very old and largely the work of the great Irish harper and composer Turlough Caerolan (1670-1738). But there was also plenty of Scottish music, a dance from Spain (Galicia, where the music "sounds very Irish," according to Barnes), and even a few dances from John Playford's anthology "The English Country Dancing Master."

This lively collection, which includes a lot of Scottish and Irish tunes, had the first of many editions in 1638, and it is still a musical Bible for lovers of country dancing. For example, you can often hear some of its tunes, in modern arrangements, played on Sunday nights in the Glen Echo Ballroom while wall-to-wall lines of dancers go through the intricate steps of traditional dances.

There were also a couple of modern pieces on the program, including one composed by Barnes and Hampton, "Starbuck's Hornpipe"--named not for the ubiquitous coffee shops or for the character in "Moby Dick" but for their pet French poodle.

Barnes and Hampton probably could make a good living as a two-member dance band if they were not so busy on the concert platform. They played their jigs, reels and other dances with a rhythmic vitality that made the adults in the audience want to dance and some of the children do it.

Considering that there are only two of them, they produced a fine variety of sounds. Barnes spent most of the evening playing the lute, an appropriate instrument for ancient dance music. He introduced it to the audience with a history of its Arabic origins and an explanation of how it got its English name, coming from the Arabic "Al aoud."

He also performed on the guitar and the Irish bagpipes (known as the "uillean pipes"), which are smaller than the more familiar Scottish ("military") bagpipes, operated with a bellows pumped by the player's elbow and capable of considerable lyric grace. Hampton played the Celtic harp--a very old instrument, as Barnes explained, "predated, probably, only by a hollow log."

They have been playing together for years, and this experience was evident in the smooth coordination and stylistic unanimity of their performance. Millennium Stage productions are offered live daily at 6 p.m., not only in the Grand Foyer of the Kennedy Center but, for those with the proper equipment, on the Internet. Instructions on how to access these concerts are available at http://kennedy-center.org or at http://www.washingtonpost.com.