“I’m entering a new thing of saying ‘no’ to stuff,” Agnes O’Casey says with a nervous cackle. “I’m scared. Every time I do it.”That sounds plausible. When, three years ago, we last spoke, the charismatic young actor could already boast a groaning CV. She had appeared in Anton Chekhov’s The Seagull for Druid theatre company. She had two films on the go. She had a leading role in the BBC series Ridley Road. Nobody could doubt that Agnes O’Casey – great-granddaughter of playwright Seán O’Casey – was on the rise, but, in this summer of 2026, she has well and truly arrived. An actor of the moment. “It’s lovely to just finally have a real run of just working,” she agrees. “It makes its way into your nervous system. You’re like: ‘Oh, cool. I’m working now.’ But even if it doesn’t take that long to shoot a film, even if your CV can look pretty stacked, there’s still a lot of time off.”She is currently in a Prague hotel room, enjoying a day’s rest from shooting the BBC’s Legacy of Spies, an indecently promising dive into the late John le Carré’s espionage universe. What does she do with such time off? There are worse places to be at a loose end.“I’ve been on so many long walks now,” she says. “I plan what I’ll have for dinner, read, and become very reliant on podcasts. Ha, ha!”Endlessly energetic, with a sharp line in creative deprecation, O’Casey, raised in Devon and north London, could hardly be better company. She has a habit of curling up into a shuddering ball of laughter when reminded of where she has found herself. In 2024, she had a recurring role as Meg Douglas in Wolf Hall: The Mirror and the Light, closing section of the BBC’s epic Hilary Mantel adaptation. In the same year, she squared up against Keira Knightley in Netflix’s spy series Black Doves and played Liz Gold in the hugely acclaimed stage adaptation of Le Carré’s The Spy Who Came in From the Cold. Her class of studied intensity is becoming nicely familiar.O’Casey is currently bossing the small screen in one the season’s most acclaimed series. Star City, for Apple TV, retells the story of From All Mankind – that streaming service’s cult hit concerning an alternative history of the US space race, from the gloomier perspective of the USSR. O’Casey plays Irina Morozova, a KGB agent who, we know from the earlier series, will, after much trauma, later become a high-ranking state official. Beginning with the Soviets beating the US to the moon in 1969, From All Mankind is rightly beloved by its fans.“I hadn’t heard of it,” O’Casey admits. “I watched it after I started auditioning, and loved it. A lot of people in my life hadn’t heard of it, but the people who did loved it. It has a very loyal and interesting fanbase. It’s been a huge relief that Star City has been accepted and enjoyed by that fanbase. You are obviously aware they are very loyal and you don’t want to let them down.”For All Mankind supposes that, if the Soviets had indeed landed first on the moon, the US, driven by humiliation, would never have stepped back from manned space exploration in the 1970s. We get moonbases, missions to Mars and a different school of rivalry between the two powers. Irina is an intriguingly conflicted character.Star City: Agnes O'Casey as KGB agent Irina Morozova “She’s a very loyal foot soldier to the regime, or at least she wants to be,” O’Casey says. “It would be naive to think that she’s questioning it. I think it’s in her DNA to sort of trust everything, and the interesting thing about her storyline is that we watch her lose faith – or have a crisis of faith.”I note that much of the series was shot in Vilnius. When we last met, O’Casey explained that one side of her family were Jewish émigrés from Lithuania. That must have felt like going home.[ Archive review: Agnes O’Casey looks like she is going to be a starOpens in new window ]“It was amazing. It was so exciting,” she says. “A few of us on the show have Lithuanian grandparents or great-grandparents. It was wonderful. Vilnius is an incredible place to be. And then there was a moment where I got excited, because there was a rumour going around that if you had a great-grandparent, you could get a European passport, but it turns out ‘no’.”Ah, yes. She misses out by just a generation or so on both sides. Shivaun O’Casey, Agnes’s grandmother (and Séan’s daughter), was born in Devon. So no Irish passport. It seems similar regulations stopped her securing the equivalent Lithuanian document.“It’s hopeless, I think. I have an Italian great-grandfather, I think, but …”My Italian passport law is rusty, but it sounds as if that might not do it either.Agnes O'Casey and her grandmother Shivaun O'Casey, daughter of Seán O'Casey in Dublin in 2019 for the official opening by then president Michael D Higgins of Flac's (Free Legal Aid Centre) head office at 85-86 Dorset Street Upper, formerly the home of the Irish playwright. Photograph: Dara Mac Dónaill O’Casey has had a lot to do with spies recently. She is also toying with a great many fiercely loyal fanbases. Le Carré’s 2017 novel A Legacy of Spies is a peculiar sort of book. Owlish genius George Smiley and his lieutenant Peter Guillam emerge from retirement to consider the betrayals and deceits depicted, 63 years ago, in The Spy Who Came in From the Cold. It seems the series has more to do with that earlier novel – famously filmed around Dublin with Richard Burton in 1965 – as it does with the later Smiley chronicle. The BBC describes the project as “adapted from le Carré’s global bestseller, The Spy Who Came in from the Cold, and drawing on material from his 2017 novel A Legacy of Spies”. O’Casey reprises her role from the theatre version as Liz Gold, a tragically misused pawn in the conflict between East and West. Hugh Laurie, Dan Stevens and Joe Alwyn also star. Matthew Macfadyen sounds like smart casting as Smiley. He can manage the same crushed intelligence. “He is brilliant,” O’Casey says. “I had a scene with him the other day, and it was pretty magic. It felt so real. You’ve done so much reading about Smiley. He is so present in all the books, and he’s so tricky. He’s cold. He’s warm. He’s calculating.”It is unusual for an actor to move from a theatrical production to playing the same character in a differently titled screen adaptation. It’s not like Barbra Streisand taking Funny Girl from Broadway to Hollywood.“Yes, exactly. I think it’s a pretty uncommon experience,” O’Casey says. “I said it to Matthew: ‘Have you ever done this?’ And he said: ‘No.’ It is wonderful. I’ve never felt so prepped. I’ve played her for so long. I’ve been with her for years, and I think that, when I finish filming, I’m going to feel bereft.”‘I was really scared, because I thought: I’m English, and no one in England knows who I am. But I don’t know if I can work in Ireland. I felt a bit placeless’— Agnes O’CaseyIt would be natural of us to assume, given her heritage, that O’Casey was always destined for a life on the stage. The story goes that Shivaun O’Casey took young Agnes to productions of her father’s plays when she was just eight. “I thought I was five, but then she said: ‘I wouldn’t have done that.’ Ha, ha! But she was the kind of person that wouldn’t really think about it. She would just bring you. And I’m glad. I’m so glad she did, because I was terrified. But I was also loving it. So it was worth it.”Did Shivaun talk much about her father? “All the time,” she says. “I think that’s one of the nicest things about having Séan as my great-granddad. I get to have a sense of connection to a great-grandparent. Whether it’s right or not, I feel I have a very clear sense of his spirit and his mind.”Indeed, born Agnes Kenig, she took his surname for her professional moniker. That feels like an implicit tribute to the author of The Plough and the Stars.“Yeah, exactly,” she says. “It felt like the right thing to do – to have the name in some way. It was completely connected to why I became an actor. Also, I like the double ‘s’.”O’Casey mentions that she is dyslexic, but it seems that was spotted early and she had the right sort of assistance.Playwright Seán O’Casey “It was pretty obvious,” she says. “Also, both my parents are dyslexic. So, I was like a nepo-dyslexic. I went to a Steiner school where reading isn’t forced upon you. It’s done very gently. You learn to read when you’re, like, seven. But it wasn’t a huge trauma for me – I guess because I was in a dyslexic household.”[ Next Year Will Be a Good One by Shivaun O’Casey: Life in the benevolent shadow of Seán O’CaseyOpens in new window ]It feels like a further tribute to her Irish heritage that, when the time came for drama school, she opted for at The Lir Academy at Trinity College Dublin. That establishment has trained a golden generation of domestic actors over the past decade or so. The likes of Paul Mescal, Alison Oliver, Zara Devlin and Éanna Hardwicke have all honed their skills there. Presumably British drama schools were also an option. “Well, they would have been if they’d let me in. Ha ha!” she says. “I auditioned for drama school three years in a row. I hadn’t heard of the Lir. And then I came to the Abbey, and someone said: ‘Have you auditioned for the Lir?’ Thankfully, I got in. I had no idea at the time how incredible the training was going to be.”She admits to being worried for the future at that stage. I can believe it. Even the most experienced actor wonders if the phone might eventually stop ringing. It takes courage to make those first tentative leaps.“I was really scared, because I thought: I’m English, and no one in England knows who I am. But I don’t know if I can work in Ireland. I felt a bit placeless.”The world soon found her. She seems as at home in Irish films such as the recent, highly acclaimed Small Things Like These as she is in high-end series such as the excellent Star City. Now skirting 30 (if she’ll forgive me), she is of an age to give advice to younger actors setting out.“I would say try your hardest to enjoy the process of auditioning,” she says. “I would also say if you’re in a room and a director is making you feel like crap, chances are they’re not a very nice person, and you don’t need to impress them. You can come across a lot of power games. I hope it’s changing, but there are these people in power with these young brains in their hands. They can be quite cruel and power gamey.”That is an uncharacteristically sombre – though undoubtedly accurate – pronouncement from a performer who otherwise radiates positivity. Pondering again, she comes at the question from a less forbidding angle.“If you’re rejected in some way, which you will be, there are a million other people that you will jive with and who will understand you,” she says. “You don’t have to be blanket accepted by everyone.”Star City is on Apple TV. New episodes every Friday.