‘Black widow.” With this code-phrase from her mum, a little girl runs to the corner shop. In the kitchen at home, her dad is attacking her mum after finding a rolled-up wad of cash she has been saving, in order to leave him. In the shop, the girl shows the owner a note: “Call 999. My life is in danger. Sandra.”

So begins Herself, the new drama from Phyllida Lloyd, the stage director who has made just two films in her three-decade career. The first, Mamma Mia!, was one of the most successful British films ever, earning more than $600m worldwide. The second, The Iron Lady, pocketed Meryl Streep an Oscar. Herself is minuscule compared with those – its budget would barely have covered Streep’s Margaret Thatcher hairspray. The film’s star is the not (yet) famous Irish stage actor Clare Dunne, who co-wrote the script and plays the young mother Sandra, a cleaner in Dublin.

After she leaves her partner, Sandra and her daughters get housed in temporary accommodation at an airport hotel with no cooking facilities (they are banned from the lobby and must use the hotel’s back entrance). They are looking at two years minimum on the waiting list for council accommodation. Then Sandra finds a video on YouTube: build your own house for €35,000.

It’s practically impossible not to shed a tear watching Herself – but not because it’s grim. Despite that harrowing opening, it’s an uplifting film, filled with acts of kindness by people, some of them total strangers. One of her cleaning clients, a retired doctor played by Harriet Walter, gifts her land and cash for materials. A schoolgate mum volunteers labour.

Lloyd has wanted to make a low-budget film for years. “I had such a curious start – going in at that Hollywood blockbuster level. I’ve been trying to climb back down to earth ever since.” Lloyd previously worked with Dunne and Walter staging a groundbreaking all-female Shakespeare trilogy set in a women’s prison. The plays originated in workshops at HMP Holloway with ex-offenders in the cast.

That experience, adds Lloyd, informed Herself: “We became so overwhelmed by the number of women in prison whose lives began in domestic violence and were fleeing abusive relationships.” The opening scene is crucial, she says. “That moment of crossing the threshold to leave is the moment when many women get killed, the moment of greatest danger. A lot of women are putting their lives and the lives of their kids at risk.”

What is unmistakably Lloyd about Herself is that it puts women centre stage and it is the work of a group of female collaborators, including Sharon Horgan, one of the producers. If you can picture Ken Loach realism mixed with Horgan’s laughs plus a teaspoon of honey stirred in, you’ve got a sense of the film’s tone.

Dunne was inspired to start working on the script a few years ago, while auditioning in New York. A friend in Dublin called to say she was about to become homeless: “She’s a single mother with three kids. She was being evicted from her house at the end of the month.” The call knocked Dunne for six. Here she was supposed to be learning lines for a detective show, but her mind was racing.

“It just felt all wrong. The world felt turned upside-down. Surely, things can be easier for her. Why is it so bloody difficult? It’s just bricks put on top of each other.” In a flash, a story came to her. What if a single mum built her own house? “No more endless forms and waiting in queues for your life to begin.” A quick Google search threw up the Irish architect Dominic Stevens, who had designed a small timber-frame house that could be made with fairly basic DIY skills. “The first thing I read was in the Guardian,” she says with a grin. “An article about Dominic building his house for 25 grand.”

Dunne spent years researching the script. She put the hours in, meeting social workers, psychologists, women’s charities, family lawyers and journalists with access to the family court. Her research into domestic violence began in a Women’s Aid charity shop in Dublin. She walked in and asked the shop assistant who would be the best person at the charity to speak to. The assistant turned out to be a domestic abuse survivor and grilled Dunne about her script. How would she end the film? Could she please show that women didn’t have to be victims for ever?

“The way she spoke was so urgent. And that was my instinct anyway. I didn’t want Sandra to be the archetype of the battered woman. These women are so brave. They’re brave when they’re staying and brave when they’re leaving.” The world of cleaning, meanwhile, required no research: Dunne’s mum worked as a cleaner until last year.

As Sandra builds her house, she recovers from her injuries, both physical and mental. Her ex begins to use contact with their kids to control her. Reading miles of transcripts from the family court, Dunne was shocked by how often women are asked: “Why didn’t you leave sooner?”

She says: “Don’t they realise that this woman has been brainwashed to a point where she has no escape. The guy has usually convinced everyone around her that everything is brilliant. There’s all sorts of gaslighting going on. Then the woman has to stand up in court and explain herself, with him sitting there, absolutely crapping herself.”

Writing the script was emotional. “I’d fucking cry the whole time sometimes. It just sort of opened things up. Afterwards, I’d feel a bit rinsed.” But by the time she came to act the scenes with violence or controlling behaviour, she felt prepared. “They were difficult on some level but I’d be building towards them.” Her performance is astonishing, and seems destined to make her a star.

Dunne never intended to play Sandra. She thought the part would need to go to a more famous actor to get the funding. Taking up the spotlight doesn’t exactly come naturally either. Lloyd used to tease her with a joke when they were rehearsing Henry IV, in which Dunne played Prince Hal. “How many Irishwomen does it take to change a lightbulb?” Lloyd would ask, then answer her own question: “Achh noo. It’s OK. Sure, I’ll be fine here in the dark.”

Cracking up as she shares this, Dunne adds that Lloyd practically had to shove her into the centre of the stage. When it came to the film, she made a condition of her directing that Dunne star. Dunne has also written a terrific part for an older actor in Peggy, the doctor who makes a gift to Sandra of land. Harriet Walter tells me how rare it is to find a sympathetic role. “I’m on a mission to tell the world that not every old person is stuck in their ways. In many plots, you’re there to be this immovable block. It’s not my experience of myself or my contemporaries.”

Dunne actually wrote part of the script while staying at Walter’s house in London, after a two-month sub-let fell through when she arrived to do a play. “I was the mad woman in the attic,” says Dunne, laughing.

All the women involved in the film seem to be upset by the news during the pandemic of the surge in domestic violence. “The burden this lockdown has had,” says Horgan, “on women in particular, and children, those suffering domestic abuse – it’s even harder to find a way out. Families on the breadline are in crisis situations. Housing is an even bigger issue now. We hope this film shines a light on these issues.”

Horgan believes lockdown has given the film an extra emotional impact: “Everyone has been isolated, with so much hardship, but there are so many stories of people reaching out, helping their community, allowing themselves to accept help. We can’t be inward-looking any more. That’s not going to work in this current climate.”

I wonder about Dunne’s friend. Did she get her housing sorted? “She stayed in temporary accommodation briefly,” says Dunne, “then with her parents for 10 months in one room – pretty hard – and eventually rented a place again. She still works like a Trojan and survives on very little. But she is doing good.”

• Herself premieres at the London film festival on 8 October and is released in UK and Irish cinemas on 16 October.