In the first few weeks of life a baby has a hearing test. If the baby is deaf “the doctors will say your child has failed the hearing test. The word ‘failure’ is associated with the birth of their deaf identity,” says Shane O’Reilly. “They have failed something in becoming who they are.”

The writer is talking about “a magical thing” that Rhiannon May said on the first day of rehearsals for Her Father’s Voice, O’Reilly’s new play. As the English actor, who is deaf, and O’Reilly, a Coda, or child of deaf adults, put it, the way such news is communicated is “the difference between seeing deafness as a culture and identity, a wholeness, or seeing it as a deficiency, a lack, a disability”.

Considering cochlear implants can pose philosophical dilemmas. It’s also meaty territory for what’s shaping up to be a big, ambitious show, with big, ambitious ideas, as well as family dramas, at Dublin Theatre Festival.

Her Father’s Voice has been seven years in the making. What started as a two-hander set in an attic is now a large play with “a far more nuanced and complicated perspective”, and with an opera and film implanted within it.

Nineteen people – actors, opera singers, musicians and the Grammy-nominated conductor Elaine Kelly – will be on stage. “It’s a complicated construction to hopefully create something very simple,” says O’Reilly, who is tweaking the script before it is printed.

A hearing couple, he a contemporary opera composer, she retraining as a doctor, and their deaf six-year old daughter, Sarah, move back in with her parents. Amid the tensions of three generations under one roof, the younger couple wrestle with the weight of their decision for Sarah to have cochlear-implant surgery. The unexpected arrival of a deaf mother and daughter unearths buried ghosts of previous deafness in the family.

As surgery looms, it morphs into the father’s opera, based on film of Sarah’s birthday party at a swimming pool. The film shoot involved “12 little girls, 47 people on set. It was wild and also unbelievably dynamic. There are deaf people, there are hearing people. Children with cochlear implants. Children who are deaf who don’t have them.

“This absolute mix, and film crews, lighting teams, a body of water. Multiple languages” – including Irish Sign Language and British Sign Language – “and people who can and cannot access the vocal score, which has to be matched with the music in the opera. It was one of those days where everyone just leaned in, and it was magic.”

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The 25-minute contemporary opera, by Tom Lane, is “a sweeping journey towards the final scene in the audiologist’s office. In the final moments the girl is switched on, and we start to hear what she hears,” the music through her cochlear implants, using technology they’ve developed to filter the playing and singing. The two young actors alternating as Sarah were born deaf and have cochlear implants.

“This is not a polemic,” says O’Reilly. “I’m not making an argument for or against anything. It’s an exploration of people trying to do what they think is best. I think people are always trying to do what’s right. But we don’t live in a world where everyone agrees what right is.”

This is all very personal to the playwright. “Growing up in a deaf household, I didn’t know we were lacking anything, because everything was there. Language, culture, identity, a sense of comfort in ‘This is who we are.’”

He describes growing up surrounded by aunts and uncles and neighbours in Churchtown, in Dublin. The house was busy and noisy, and he and his sister, Aoife, were noisy kids. Deaf people would show up unannounced (phone calls being impossible).

“And suddenly there’s 10 deaf people, and a ferocious evening of catch-up and chat. People think deaf households are quiet. Deaf people are very vocal.”

The old term “deaf and dumb” is very insulting, says O’Reilly. “It insinuates these people do not speak, whereas everybody is attempting to engage with the generation of sound and speech. When I’m at home now, my mum says something across the kitchen. I understand exactly what she’s saying. And now my husband understands.” His husband is Paul Curley, an actor. “We chat away. She’s always roaring and screaming at him, and he’s laughing his head off.”

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O’Reilly’s mother, Susan, was born profoundly deaf. His father, Bernard (or Waffo), lost his hearing to meningitis, “so has some access to the world of sound” and wears hearing aids. She’s a special-needs assistant at Holy Family School for the Deaf, in Cabra in Dublin. He’s a carpenter on Francis Street, the city’s antiques quarter.

O’Reilly recalls friends meeting his deaf parents when they were younger. “Some people were very uncomfortable, just wanting the interaction to be over.” Sometimes it was like “a lack of respect for a parent. I was raised that if you go to your friend’s house, you say ‘Hello’ and ‘Thanks for having me’. That was not always afforded to my mum and dad.

“But one of my best pals from college – he’s from London – I could have cried. Straight in: ‘All right, then, Mrs O’Reilly!’ We’re going to make this work. Kept going. He didn’t care that it was uncomfortable. He’d learned that discomfort is an opportunity.”

This seems key to O’Reilly’s connections within and without the family.

“I spent so much of my childhood interpreting everything and feeling I must make everything okay, make sure everybody’s understood and that nobody’s offending anybody.” When friends lean in, “I have this immediate sense of calm.”

The deaf community getting together, “like immigrants here, or the Irish abroad”, have “that sense of, ‘We’re together, and we know each other intimately.’ We had that going to the deaf club in Cabra. Mum chatting away to all her pals. I can sign with the deaf kids.

“We were always included, as the hearing kids of deaf parents. And we’re all flying around, the deaf and the hearing kids all together. We had a football team, half-deaf, half-hearing.” That openness taught him a lot subconsciously, as a child.

He’s aware of his parents’ generation spending their lives in a system, in the 1950s, 1960s and 1970s, “where Ireland was very hostile towards deaf people. Those schools were not easy places to be and had a dark history. They’ve come through that, and they’ve pushed into the workforce. They’re making lives, they’ve bought homes, they’re raising children.”

When cochlear implants first became available, many deaf people regarded “this announcement, with celebration, that their deaf children can be made into hearing children” as a rejection of their culture. Doctors were threatened. “The deaf community felt like this was an attack.”

“Now we have an empowered deaf culture, where sign language comes first, and a lot of the deaf community’s relationship to cochlear implants is, ‘Absolutely get a cochlear implant, but don’t stop signing, and do not take the child out of the deaf culture. Continue to give them access to everything.’”

O’Reilly understands a hearing person with a deaf child wanting their child to access and engage with the world the same way they do. But if the child already has access to an empowered culture, “then the dilemma becomes deciding between two things, as opposed to a problem and a solution”.

He talks about the drive to cure, to improve. “This is an awful thing to say, but I wonder what society would be like if there was a drug you could give teenagers to stop them being homosexual”, which was long considered dysfunctional.

Early in the research, he and Lane, the composer, spent time with the cochlear-implant and audiology team at Beaumont Hospital. “They’re so passionate about this as a positive thing, as an opportunity to help children access something.”

They explained how the human brain starts a relationship with a piece of technology, and how it’s different for everyone. “This is about your brain chemistry.” The “unknownness of it” interested O’Reilly. “Fundamentally, we don’t know whether it will work for your child or it won’t, whether emotionally, in the future, they will feel happy with this decision” their parents made.

A line in the play comes from doctors describing the little hairs inside the cochlea transmitting sound, which is how we hear. Even deaf people have some functioning hairs. Before surgery, they’re destroyed. “The doctor described it as driving a JCB through a daisy field.”

If he had a deaf child would O’Reilly opt for surgery? “I have no idea.” He’s not deflecting. He observes how everything changes when a baby is born. “Priorities are immediately stacked in a different order.”

He has “very proud deaf friends” who’ve chosen implants for their children, to give them options. He’s also aware some people’s experience is not good, and some later regret their parents’ decisions.

“Not everything I make is about the deaf community,” says O’Reilly, but he does keep coming back. He wrote and performed in Follow, with WillFredd Theatre Company, in 2011, “performing in sign and speech, so my parents and their friends could see something I was in” without having to look right, off the stage.

He wrote Window Pane for the Abbey Theatre’s Dear Ireland project, with Amanda Coogan performing behind a pane of glass, and the short film Pal, with a deaf cast. “I’ve written a lot of work representative of the deaf perspective, coming at the world from that lens I grew up with.”

Today is O’Reilly’s last in rehearsals. “It feels the stakes are high. I feel very protective of that world, of the narrative around it, of my parents, that community.”

For his parents, “I think it makes them proud. Because this was not the way they grew up. People were not proud of their deaf children. People were not proud of the noises deaf people made, of the spectacle of sign language. So for their son to repeatedly and endlessly talk about it, I hope for them there’s a catharsis.”

Her Father’s Voice, which is running as part of Dublin Theatre Festival, previews at Draíocht, Blanchardstown, on Saturday, September 27th, and at O’Reilly Theatre, Dublin, on Wednesday, October 1st. It is then at O’Reilly Theatre from Thursday, October 2nd, to Sunday, October 5th