Rebecca Ryan is pregnant again.

At just 21 years old, the former star of the council estate comedy drama, Shameless, has had more buns in the oven than most. The last time was during a two-year stint on TV in Waterloo Road, in which Ryan's character, schoolgirl Vicky McDonald, became pregnant. Before that Ryan played a pregnant runaway in Laurence Wilson's stage play, Lost Monsters. Now it's the big one, as Ryan prepares to play Jo, the lippy Salford teenager in Shelagh Delaney's 1958 play, A Taste of Honey, at the Royal Lyceum Theatre, Edinburgh's new production of this somewhat neglected play.

Ryan struts around the rehearsal room as Jo, tearing verbal chunks out of Lucy Black, who plays her slatternly mother, Helen, her cardigan stretched by the pillow-like appendage stuffed under it. Watching Ryan, it could be an older version of Debbie Gallagher, the youngest of Shameless's tempestuous Gallagher clan brought vividly to life by writer Paul Abbot. Ryan played Debbie for six years, being cast in the programme when she was just 11. Once rehearsals are over, Ryan couldn't be more different from the troubled adolescents she has become so adept at playing.

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"Jo's learnt to take care of herself, and is very independent," she says. "She'd have to be with a mother like Helen. She's brought herself up, basically, and has gone weeks without Helen being there. They have a very fiery relationship, but as much as she says in the play she hates Helen and doesn't want to end up like her, she is becoming her, because it's all she knows. She doesn't know any other way, so as much as she detests how Helen lives, she ends up becoming exactly the same."

There was an example of this earlier, when Jo's gay best friend Geoffrey, played by Ryan's brother Charlie, tried to break up Jo and Helen's slanging match, only for the pair to gang up on him.

"That's how they cope," says Ryan. "In a way it's like in families, when I can call my brother something, but no-one else can. That's Helen and Jo's relationship, and that's how they fight. They're at each other's throat one minute, then the next one of them is asking what's for tea."

Delaney, who died in 2011 at the age of 73, famously wrote A Taste of Honey after watching Terence Rattigan's Variations On A Theme at Manchester Opera House. She was so appalled at what she saw as Rattigan's insensitive treatment of homosexuals that she wrote A Taste of Honey in a precocious 10 days. The play was accepted by Joan Littlewood's Theatre Workshop company, and became a taboo-busting hit, with race, class, sexuality and gender all coming into play.

A more naturalistic film version directed by Tony Richardson was released in 1961. This proved to be hugely influential, not least on Salford-born Smiths singer, Morrissey, who lifted many of his early lyrics from Delaney's play. The Smiths' song, This Has Opened My Eyes, was her narrative in miniature.

Delaney's follow-up to A Taste of Honey, The Lion in Love, received a lukewarm response, and she wrote fiction and screenplays, fading from view until her death.

While neglected, A Taste of Honey paved the way for other working-class female writers, including Andrea Dunbar, who wrote her first play, The Arbor, at 15 before penning Rita, Sue and Bob Too. It's easy to see a lineage that stretches to Jim Cartwright's The Rise and Fall of Little Voice. It's even arguable that Shameless couldn't have existed without A Taste of Honey.

Given her age, Ryan was unsurprisingly only vaguely aware of the play before she signed up for Tony Cownie's Royal Lyceum production, but recognised its backdrop from the off.

"It's really hard-hitting and grabs you straight away," she says." It's quite an intense piece, which is great to get your teeth into. It must have been quite hard-hitting and shocking at the time it came out, but I think it is even now. You know a lot more about the things that happen in the play now, but the relationships between them on stage are just as shocking. The things that happen in the play are just as bad as when they were written, so it really stands the test of time."

Ryan was born in Prestwich, a Manchester suburb a stone's throw from the Salford backstreets where A Taste of Honey is set. While by no means a showbiz family, Ryan's mother had been a world champion Irish dancer, and Ryan looked set to continue the tradition. Charlie had become interested in drama at school, and had worked on television as a child actor from an early age.

When she was six, Ryan accompanied her brother to an audition for The Who's rock opera, Tommy, at the same venue where Delaney had seen Rattigan's play. She ended up being given the part of the young Tommy.

Despite enjoying the experience, Ryan drifted back to her dancing until Charlie got a part in Paul Abbott's prescient political thriller, State of Play, playing the son of David Morrissey's character. The producers had yet to find anyone to play the younger sister of Charlie's character, and he let slip that he actually had a little sister who might fit the bill. The pair auditioned together and Ryan got the part. This led directly to Abbot asking her to audition for Shameless, which changed everything.

"I was just going into high school," Ryan remembers, "so because I was so young it was just what I did and I got on with it and everyone I knew just accepted it. Everybody loved the programme, so that helped, and it was such a family unit there, that was when I realised how much I loved it, and couldn't imagine doing anything else. I wouldn't be here now doing this if it wasn't for Shameless. It opened every door it possibly could, and everything that's happened to me since is because of it. I loved every second of it."

While working alongside David Threlfall, Maxine Peake, James McAvoy and Anne-Marie Duff was all the acting education she needed, Ryan left Shameless shortly before her 18th birthday. She made her stage debut at the Royal Court in London in Fiona Evans's Edinburgh Festival Fringe hit, Scarborough. After A Taste of Honey, Ryan would like to diversify beyond playing council estate mums. "I'd love to get into Downton Abbey," she says. "My other favourite is Smash, but I'd have to play a non-singing part as I can't sing. I'd just like to show people a different side to me."

A Taste of Honey, Royal Lyceum Theatre, Edinburgh, January 18-February 9.