It was going to be an album track: Charles Byrne Is Dreaming – but somehow, like this song’s protagonist, the music outgrew itself. My three-minute number evolved into a 90-minute opera, which opened the Aldeburgh festival last summer and is coming to the Linbury theatre at the Royal Opera House this week.

This outcome would have astonished me in the early 1990s when I had my first, conflicted encounter with the skeleton of Byrne, “The Irish Giant”. I was working as a curator in the Science Museum, London. At the end of the day, I found it eerie and overwhelming to step through the darkened galleries, the gloom punctuated by countless artefacts. On a visit to the Hunterian Museum I saw thousands of human specimens prepared and preserved in the 18th century by surgeon and anatomist John Hunter – a child in the uterus, the branching vessels of a lung, a cancerous growth gnarling a femur. These exquisite human structures were finely wrought examples of the anatomist’s art. Hunter learned about the body by looking, smelling and touching, and I’m one of many millions who have benefited from his research. But there was an uneasiness in seeing his collection – these tissue specimens held traces of many past human tragedies. I wondered how it felt to be among these artefacts after hours with their complex biographies crowding out the night.

It was on this visit that I first saw Byrne’s skeleton. Accounts of his height vary but he was at least 7ft 7in tall (2.3 metres). Byrne, who died in London in 1783 aged 22, spent his short life exhibiting himself to the paying public. Onlookers were in awe of his extraordinary physique – the result of an undiagnosed pituitary tumour. I too was captivated as I stared through the glass at Byrne’s long femurs and expansive ribcage. I relished looking at Byrne, the specimen, until curator Simon Chaplin told me the lengths this man had gone to in order to avoid our gaze. Byrne was so determined not to be cut up and exhibited after death that he paid undertakers to seal his corpse in a lead-lined coffin so his friends could drop him into the sea at Margate. But Hunter arranged for someone to intercept Byrne’s funeral procession and steal his body. By the time it was dropped into the sea, Byrne’s coffin contained only rocks.

When I considered Byrne’s fate, my mind turned to a very different funeral, at the end of Venus and Adonis, John Blow’s masque composed a century before Byrne’s death. In one of the most rapturous moments in English baroque music, the chorus sings to Venus: “weep for your huntsman” after she keens, grief-stricken, over Adonis’ body. I wondered how it would feel to give Byrne his own funeral music, to affirm him as someone beloved, ready to be buried with the dignity of a prince. I imagined music rewinding his story, undoing certain acts, and so the seed for Giant was planted.

The odds of me writing this debut opera were slim – I’m well past 50, in a contemporary classical world that has a vampiric fixation with youth. And although I’ve been playing and notating scores since childhood, I’m primarily an electroacoustic composer. The essential descriptors of my soundworld are waveforms and spectra as much as notes on a stave. It’s likely my plans for Giant would have got no further than Bandcamp had I not stumbled on a scheme run by Britten Pears Arts and the Jerwood Foundation. They were offering time and money for small teams to come up with a few minutes of new opera from scratch. This enabled me to bring in poet Ross Sutherland and director Sarah Fahie to create Giant with me in Snape Maltings among the reed beds of Suffolk. I called Ross after seeing him perform Standby for Tape Backup – an achingly beautiful live poem about memory, death and re-runs, set to fleeting images from a messy family VHS tape. In Giant, we were also bringing sound and image together. We had a complex story to tell – and we needed to embrace nuance and moral uncertainty while finding empathy for both Byrne and Hunter. And then there was the small matter of presenting Byrne, the giant, himself. Given his history, would it be disrespectful to depict him? Would anyone understand this story if we didn’t?

Director Sarah Fahie’s first instinct was to put the story on its (very large) feet. As we shimmied around Snape Maltings, certain essential flashbulb moments emerged. The most striking didn’t depict Byrne himself but Hunter. We showed him alone at night at the autopsy table, emotionally conflicted as he dissected the body of his own infant son – an event based on historical record.

I started with the soundworld of that imagined autopsy scene. I knew I was seeking something crepuscular, and found that in pianissimo sounds of a plucked viola da gamba – an instrument often associated with the tomb. I wanted to hear the metal of the surgeon’s blade and flecks of sound on the edge of perception, like motes of dust in the air. John would be singing introspectively – he was steadying himself. I wanted to sonify that tension between wanting to look and wanting to recoil that runs throughout this story – so I made electronic reflections of gamba’s gestures that diffuse through the air.

Sonically I’m drawn to the uncanny – fittingly, one translation of Unheimlich is “unconcealed”. I felt as though the score could anatomise sound itself – something reflected in Hyemi Shin’s design and Adam Silverman’s lighting which enable audiences to glimpse the workings of the show.

My spectral shimmers and extrapolations may have been promising in the studio but in the rehearsal room they made for a bumpy ride. Music of this nature makes little sense as a piano arrangement, it took nerves of steel not to buckle when the rather brilliant singers heard the score for the first time, reduced to a rehearsal room piano.

I was relieved and thrilled to hear seven instrumentalists play the full score, just a few days before the first performance. And I’ve been overwhelmed by the beauty of sound the singers bring to the piece. It feels as though Giant could always be in development, taking on insights from the performers who are closest to it.

How did we present Byrne? Our first step was to scour the world for a charismatic and very tall tenor. Lebanese-American singer Karim Sulayman was perfect for the role, with a fine voice that’s also very well suited to the music’s baroque and electronic timbres. Height-wise, Karim is assisted by the set and some deft costume design by Nicky Gillibrand.

We’ve also introduced a spoken role – Hunter’s grave-robbing assistant, and three female voices: Rooker is an impresario with an ambiguous relationship to Byrne, and two museum curators, our witnesses – ghosts from the present who walk through the literal bones of the story and hold it to the light.

I can count the hundreds of days and nights it took to craft every beat of this story. Walking along Bow Street this week, on my way to tech rehearsals, I caught sight of the poster for Giant on the side of the opera house for the first time. It felt extraordinary – as though together, against the odds, we’d made a rocket in the garden and landed it on the moon. Every day I wonder what Byrne and Hunter would make of it. I hope if they saw Giant, they would have both felt understood.

• Charles Byrne’s skeleton remained on display until 2017 at the Hunterian Museum. Giant is at the Linbury theatre, Royal Opera House, from 8-15 March.

• This article was amended on 12 March 2024 to correct the year that Charles Byrne’s skeleton was removed from public display.