Boys of the Lough. Most people plan their concert schedule on a weekend to weekend basis, ignoring the oceans of music that wash over Cambridge each Monday to Friday. Next Tuesday and Wednesday, the Boys of the Lough, four musicians from Great Britain and Ireland, will play at Passim. If you know you are going to be busy one of those nights, take care of your work this weekend. These four are among the world's very finest folk musicians, and I can safely say you will be astounded by their performance.

Aly Bain, their fiddler from the Shetlands, produces the music of what sounds like an orchestra of violins. A serious student of this century's champion fiddlers throughout Britain, he is able to recreate, for example, a dazzling fiddle tune in half a dozen incarnations representing different fiddling traditions. Cathal McConnel and Robin Morton, their champion flutist and percussionist, have rich, lilting voices to match their meticulous musicianship. The group's original guitarist, Dick Gaughan, has been replaced in the last year by, I believe, another Scot or a Welshman. Together, they can call upon an amazing variety of musical backgrounds.

Ordinarily, extended recommendations run the risk of raising hopes so high that a performance cannot possibly match people's expectations. Don't worry about it. I have understated the case for hearing this group. Tuesday and Wednesday, March 26 and 27 at Passim. Call 492-7679 for info.

Music notes. Local favorite Tom Rush plays The Performance Center this week through Saturday, March 23, 8 and 10:30 p.m.... Grace Slick and Paul Kantner steer the Jefferson Starship into the Music Hall Wednesday, March 27 at 8 p.m.... Brandeis presents a mixture of glittery soul and local boogie when lke and Tina Turner meet the Chris Rhodes Band on Saturday, March 23 at Shapiro Gym, Brandeis University, at 8 p.m. Call 661-3877 for info.... The legendary Preservation Hall Jazz Band moves to Symphony Hall this Friday, March 22 at 8:30 p.m.