Kathleen Ferrier's celebrated rendition of Gluck's aria, Che faro senza Eurydice/What Is Life To Me With- out Thee?, was a bit puzzling on first hearing. Who was this passionate female lover of Eurydice and what had happened to Orpheus? What Tommy O'Brien's Your Choice and Mine hadn't clarified was that the tradition of a mezzo soprano singing the role of the grief-stricken Orpheus in Orfeo ed Eurydice wasn't just an arbitrary bit of operatic crossdressing: Gluck in 1762 originally composed the central role for a castrato, and, in his expanded French version, for a very high tenor (haute contre).

But by the time Berlioz came to rework the opera in the mid19th century, both these voice types were almost extinct, and he rearranged the part of Orpheus for a mezzo. Since then Ferrier, Janet Baker and recently Anne Sophie Von Otter have performed it superbly, and this week Welsh National Opera brings its touring production of the Berlioz version of the opera to Belfast, sung in French with the Swedish mezzo, Katerina Karneus as Orpheus.

This company, under the direction of Patrice Caurier and Moshe Leiser, has tackled Gluck before, very successfully, with Iphigenie en Tauride. But Orfeo ed Eurydice, the 18th-century Bavaria-born composer's most appealing work, presents more of a challenge. The elements that were deemed problematic in the 19th century and kept it on the margins of the repertoire - the inclusion of extended ballets between the acts and the emphasis on spectacle rather than plot - work in its favour today. As live performance becomes increasingly inter-disciplinary, moving towards an integration of dance, design, video and installation art, text, music and drama, Gluck's choreographed tableaux offer exciting possibilities.

His reforming mission also has great 21st century appeal: the emphasis is on "beautiful simplicity", emotional depth, serene melodies, joyful dramatic expression. He wanted to create an azione teatrale per musica (theatrical action for music), to revitalise an artform which he considered had become stultifyingly formal and stylised in the previous century. In Orfeo, the impassioned, exhilarating choruses play almost as significant a role as that of Orpheus, and the recitative is given fluid, uninterrupted musical accompaniment - a marked change from the dribbling continuo of earlier Baroque opera that tended to grind things to a halt.

All this, of course, is supplementary to the subject matter of Orfeo, with its great thematic triangle of love, death and art, its haunting myth of Orpheus, the Thracian poet and singer with quasi-divine powers, who charmed all living creatures and moved rocks and mountains with the sweetness of his music. For 2,500 years, from Simonides to Cocteau to Kenneth McLeish, the story of this archetypal artist-poet has been adapted and recreated in Western painting, music, literature and most recently, film, with particular emphasis given to his descent into the Underworld to rescue his beloved bride, Eurydice, who had been fatally bitten by a snake. This episode, based on Virgil and Ovid's accounts as adapted by the librettist Calzabigi, is central to Gluck's opera, which opens with Orpheus mourning Eurydice's death.

Visits to the realm of Death, the Underworld, in an attempt to assuage intense personal loss had also been undertaken by Odysseus, Heracles and Aeneas, and they echo ancient fertility myths. But Orpheus's descent is one of the most spectacular in classical literature. His music takes the Underworld by storm, bringing the wheel of Ixion to a standstill, stopping the vultures from pecking at Tityus's liver, making Sisyphus sit on his boulder to listen and making Tantalus forget his thirst. Zeus agrees to let Orpheus bring Eurydice back to life, on condition that he doesn't turn to look back at her until they have reached the earth. He agrees, but breaks his promise just as the end of their journey is in sight and Eurydice dies.

Whether Orpheus looks back through an excess of love, or loss of faith and fear that she's not following him is uncertain. In the opera, he looks back because Eurydice is audibly distraught at his apparent coldness and loses faith in him (nobody had explained the plan to her, obviously). Griefstricken for the second time, Orpheus sings the lament Che Faro senza Eurydice?/J'ai perdu mon Eurydice so heartrendingly that Amore (Eros), the obliging deus- ex-machina, allows the lovers to be miraculously reunited for an encomium to the power of love.

It's fitting for an opera, of course, that the musical arts should triumph over death. But the Orpheus story had less joyful outcomes in other versions, and has been endlessly adapted in transmission. In fact, the closer we look, the murkier Orpheus becomes. While his story was the basis of an influential religious movement called Orphism (dating from the 6th century B.C.) which emphasised self-denial, physical purity and the transmigration of souls, it's not clear whether the adherents of this mystical sect considered Orpheus to have been a real person.

He doesn't feature at all in the Greek epic tradition (Homer and Hesiod) and lies outside the main framework of Greek myths. Variously described as the son of Apollo or of a Thracian King, whose mother was one of the muses, his name was included among the Argonauts on Jason's voyage - along with almost every other significant Greek hero. In a lost play by Aeschylus, Bassarae, his violent death at the hands of female followers of the god Dionysus (Bacchae) was recounted. His dismembered body floated in the river Hebrus, while his severed head, still singing, floated to Lesbos, where it acquired oracular powers. (This part of the story features in Harrison Birtwistle's 1988 ambitious opera, The Mask of Orpheus.)

Their motives for the gruesome attack on Orpheus have been exhaustively interpreted. Was it revenge for his rejection of their advances, his devotion to the memory of Eurydice, or his sexual preference for men? Or was it because he rejected Dionysus and founded his own cult?

The main ancient sources for the myth are Virgil, Ovid and Seneca, which were later copiously annotated by medieval scholars keen to recruit Orpheus to the Christian tradition and to associate him explicitly with the figure of Christ. In medieval iconography allegorical interpretations of the visit to the Underworld proliferated, fusing classical and Christian imagery and connecting it to Christ's Harrowing of Hell and the Resurrection.

The motif of the hero's quest, a journey through a realm of darkness, through trials of morality and bravery recurs in European literature and has profound emotional, psychological and religious resonances, relating to the Christian idea of the dark night of the soul, to despair and to Purgatory. Orpheus's story also adapted well to the conventions of courtly love as seen in the anonymous 14th-century, Middle English poem, Sir Orfeo, while the Romantics took a keen interest in the virginal Eurydice and focused on the tragic aspect of the story, the impossibility of attaining perfect love . . .

It's clear that there have always been two antithetical versions of this infinitely adaptable myth: the first portrays the singer-poet transcending physical limitations - including death - through his art, and this has attracted attention from writers on myth such as Mircea Eliade, who are interested in spiritual powers and shamanism. The tragic version, explored in depth by Rilke in his great sonnets, is of the grieving, all-too-human Orpheus, defeated by death, plunging into the depths and re-emerging bereft of his lover/muse, but transforming his suffering and longing into art. Gluck and his librettist took it all much more lightly: they had no difficulty collapsing these oppositions and giving us both versions at once: an extraordinary outpouring of grief, followed by glorious triumph. That's opera for you . . .

Orpheus and Eurydice plays at the Grand Opera House, Belfast, on Wednesday and Friday of this week. Booking: 048 9024 1919.