Breton music - particularly but not exclusively that of the Celtic harp wizard Alain Stivel - strikes a specially responsive chord in many American audiences, and the reason is not hard to figure out. Brittany is a final outpost of the Celtic culture that once prevailed through a large part of the European continent before being pushed into its small corner by the encroachments of the Roman legions and the Germanic tribes.

It was the culture of King Arthur and St. Patrick, not to mention the Druids, and its megalithic monuments at Stonehenge are echoed in similar structures - menhirs and dolments - that add mystery to the landscape of Brittany. That culture came to our shores with earliest European settlers and we were inundated with it a century ago after a famine sent the sons of Ireland around the world looking for new homes.

So it should not be too surprising that a quartet of Breton musicians called An Triskell, which specializes in contemporary popular adaptations of old Celtic music, sounds like it would be completely at home playing in Matt Kane's.Or that one of the instrumental numbers on its new import record, Musiques Celtiques (Philips 6332 145) turns out to be identical with a popular American square dance tune, "Donkey Reel." The Celtic culture transcends national boundaries, and the material on this record, drawn chiefly from Brittany but also from Belfast and Glasgow, Dublin and the Hebrides, demonstrates an underlying compatibility in the various flavors of Celtic music.

The four musicians in An Triskell sing both in Gaelic and a rather accented English, and they handle a dozen instruments, among them the Irish harp and Irish whistle as well as the more familier guitars, flute, banjo, mandolin and violin. Stivel fans looking for more of the same may find this record problematic,; there is obviously a connection in the roots of the material, but Stivel produces a more ethereal king of poetry that contrasts with the earthy, strongly rhythmic approach of this quartet. Still, I find myself playing this music over and over again, with constantly renewed delight in its color and vigor.

An important record completely different in style but perhaps even stronger in its appeal to some tastes is a collection by Amalia Rodrigues: Fados e Guitarrados au Portugal (Festival 113, two records). The fado is Portugal's answer to the French chanson, the American torch song and even some kinds fo Flamenco - music of great beauty, intensely personal and sometimes a bit Middle Eastern in its cadences.

Rodrigues is its greatest exponent in our time, and this collection gives a fine sampling of her art, but it is also puzzling. The first of the two records is a well-integrated production, closely focused on Rodrigues with traditional guitar accompaniment throughout. The second record has four numbers by Rodrigues, obviously recorded at another time with orchestral accompaniment. The remaining of the record is filled out with guitar solos that are very well performed but have nothing noticeable to do with Anakua Rodrigues. Despite evident padding to fill out two records, however, this is an excellent production for those who enjoy the unique Portuguese flavor in popular music.