This week's National Symphony concerts emphasize the intention of Mstislav Rostropovich to focus the spotlight on distinguished young artists. He will conduct three. Mark and Alexander Piskunov, violinist and pianist, and cellist Yo Yo Ma, in solo concertos by Saint-Saens. Wienlawski and Liszt, putting them all together at the end of the evening in the Beethoven Triple Concerto.

Tonight at 8 o'clock, the Colmcille Women's Choir of Derry, Ireland, will sing a benefit concert for Ireland's Children, Inc. The program will be given in St. Matthew's Cathedral. Tickets will be on sale throughout the day in the lobby of the Mayflower Hotel, and at the door prior to the concert. The program will be of Irish music.

Music from Mariboro, the chamber music series that brings with it the flavor of the music-making at the famous music center in Marlboro, Vt., returns to the Smithsonian's Baird Auditorium at 5:30 p.m. Saturday. At that time, the musicians will play Beethoven's Octet, Gounod's Petite Symphonie, and Mozart's Serenade, K. 375. The concert is the first in a series of three, the other two of which will be played on March 22 and April 29. Tickets are available through the Smithsonian central box office. Phone orders are accepted at 381-5395.

At noon on Wednesday, Francis Rizzo and Gary Fifield will discuss the season of the Washington Opera which opens on Nov. 11 with Mozart's "Magic Flute." Rizzo is artistic administrator and Fifield managing director of the company. The symposium, the Kennedy Center's weekly gathering, is free to the public. It will be held in the AFI Theater.

At the conclusion of the Beethoven Society's recent Opera Singer Competition, the judges felt compelled to split the prize, awarding $500 cash to soprano Mary Jane Bane of Takoma Park, Md., and mezzo Adelle Nicholson of Chevy Chase, Md. Second prizes of $250 each went to soprano Linda Mabbs of Gaithersburg and tenor Dana Wayne Talley of Oxon Hill.

The U.S. Air Force Symphony Orchestra will begin a six-week series of concerts to be given in the National Presbyterian Church on Wednesday and Nov. 9, 16, 22 and 30 and Dec. 7. The concerts will be at 8 p.m., and are free. Col. Arnold Gabriel will conduct the opening concert Wednesday, with music of Ibert, Telemann, Respighi and Berlioz.

Raissa Tselentis, founder of the Bach International Competition, has put together a new contest for junior pianists. It will be held on May 6 and 7 in Krannert Center on the University of Illinois, Champaign campus. It is jointly sponsored by the National Academy of Arts and the Youth Concerts Foundation, both of Champaign. The junior division of the new competition is for pianists between the ages of 6 and 10; the intermediate division for those between 11 and 13. Details on the competition, in which study grants totaling $4,650 will be awarded to the winners, are available from Philip Faucett, Youth Concerts Foundations, c/o National Academy of Arts, 17 East University Ave., Champaign, Ill. 61820.