Lockdown, went one joke on Twitter, was like being in a play by Enda Walsh. The closed worlds of the Irish dramatist could have been made for the pandemic. His characters don’t need a government edict to stay at home; they are too ensnared by their own warped logic to do anything else. Walsh takes introspection to feverish, funny and surreal extremes. His breakthrough hit, Disco Pigs, was about two teenagers who spoke only in a private language. The Walworth Farce imagined a family of Irish expats endlessly re-enacting a play in their front room. Ballyturk took place in a hermetically sealed house where two men invented the life of the town outside. So even though he wrote Medicine before Covid-19 struck, we shouldn’t be surprised if it resonates with recent experience when it premieres in the Edinburgh international festival next month. “Enda’s got copyright on Covid-19 and is making a ton of money,” he mocks after a day’s rehearsal, a stage manager disinfecting chairs behind him. The joy of talking to Walsh is he finds his plays just as odd as we do. He laughs with incredulity as he describes the eccentric rituals of his characters. In the case of Medicine, they are two musical-theatre actors, played by Clare Barrett and Aoife Duffin, who work in a psychiatric institution where they role-play the life story of their patient, John Kane (Domhnall Gleeson). It’s some kind of therapy but, as they edit his story down to the best bits, their motives are suspect. Kane, meanwhile, has found himself in the system almost by accident and now can’t get out. “I’m always looking for characters who feel they are on the edge of everything, but here I’m talking about a person who, through our eyes, has mental health issues,” says Walsh. The playwright has not suffered mental illness himself, but has close experience of it. “All of the plays have been about what happens when a person hasn’t been loved or looked after properly. This play is about people who are in care, in an institution or are addicts – people who need looking after – and examines what happens when we neglect them.” He adds: “For me, theatre is a way of having a good look at myself and saying, ‘Am I still a human being? What sort of a human being am I? How do I rate my humanity?’ This is a play about that.” If the worlds Walsh creates are appealingly odd, equally curious is his relationship with the mainstream. Though his plays have enjoyed great popularity on the festival circuit (“They’re busy, loud, entertaining and foolish”), they hardly pander to the commercial market. Yet Walsh also has his name on the stage musical Once, with its eight Tony awards, and the Roald Dahl adaptation The Twits. His version of Max Porter’s Grief Is the Thing With Feathers featuring Cillian Murphy drew in an audience of Peaky Blinders fans and sold out instantly. If it hadn’t been for the pandemic, he would have opened a musical, Sing Street, on Broadway by now. Walsh, who wrote Lazarus for David Bowie and Ivo van Hove, keeps some pretty starry company. That includes Gleeson, veteran of Harry Potter and Star Wars movies. Thanks to the closeness of the Dublin theatre scene, the playwright has known him since he was a boy. “In my mind, he’s just a guy I’ve known a long time,” he says. “That’s the thing about all these actors I know – I forget they’ve done movies.” In 2015, Gleeson appeared with father Brendan and brother Brian in a revival of The Walworth Farce directed by Sean Foley, a production Walsh describes as “ridiculously funny”. The playwright wrote Medicine with its four actors in mind – Sean Carpio completes the cast – and then went after them. “I just did it. I said, ‘I’ve written this part for you. You cannot say no!’” By rights, a CV that includes Hunger for Steve McQueen and an up-and-coming project for Netflix shouldn’t also stretch to a play billed as “deconstructing the fabric of theatrical performance”. Even Walsh seems to agree. “When people see my work and like it, they will approach me to work with them and start pitching me commercial-type ideas,” he says. “They’re intrigued by the way I’m wired. But certain things, I have to fire myself. I just go, ‘I can’t deliver that. We can remain friends, but we don’t have to work together.’ Things like Medicine are where I’m most at home, but I adore doing the films and musicals, too.” Medicine is at the Traverse as part of the Edinburgh international festival, 4–29 August. Then at Black Box theatre, Galway International arts festival, 2–18 September, and on demand, 20–26 September.