Daniel Barenboim's interpretation of Richard Wagner's Tristan und Isolde (itself an interpretation of Shakespeare's play) opens this week at the Metropolitan Opera. The five-hour opera features Katarina Dalayman and Peter Seiffert in the leading roles, with Michelle DeYoung as Isolde's maid, Brangäne. Although sung in German, the characters depicted are Irish and English. In fact, Isolde and Tristan begin their tempestuous relationship over a question of Irish pride. The former enemies fall in love, but before they can consummate their relationship, previously arranged betrothals must be canceled. The minimalistic, monochromatic background color scheme beautifully compliments the simple costumes designed in royal yellows, blues, and greens.