As TV debuts go, last year’s Derry Girls was nothing short of phenomenal. The coming-of-age comedy followed four teenage girls from Northern Ireland and their witless sidekick James as they navigated both minor aggravations and Troubles-inflected dramas in mid-1990s Derry.

It became an instant classic and was picked up for another season before its second episode had even aired. The show, which returns to Channel 4 next week, averaged 2.5 million viewers across the UK, including half of all TV watchers in Northern Ireland. Unsurprisingly, it has changed the lives of its titular girls and “that wee English fella”, as Dylan Llewellyn will now for ever be known – much to their various families’ delight.

Half of Derry now turns up at the shoots, says creator Lisa McGee, ‘and they'll tell the dogs off for barking’

A recent trip to the garden centre left Nicola Coughlan – who plays nervy Clare, who came out at the end of the first season – wanting to die quietly behind some potted plants. Her mother (“She’s, like, the proudest mother in Ireland”) joked to the cashier about being surprised her daughter was helping her bag their purchases – “cos she’s FAMOUS!” When the cashier didn’t laugh, her mother repeated the joke, only louder. “Oh, kill me now,” thought Coughlan.

On shoots, says the show’s creator Lisa McGee, what seems like the entire population of Derry now gathers around, not so much to point and gawk but to keep guard – and even “tell the dogs off for barking”. Given all this, it isn’t hard to see where the show’s domestic authenticity comes from.

I meet some of the team in London, after a screening for the second season. At the recent premiere in Derry, they tell me, there was a Q&A, but the only people who managed to get any questions in were friends and family. This close-knit environment reminds me of the show’s start, in which one girl reads another’s diary aloud: “Everybody knows everybody, knows everything about everybody, and sometimes all I really want is to be simply left alone.”

‘People might like this’ … creator Lisa McGee. Photograph: Channel 4

Listening to them rattle off their stories is a bit like watching a Tina Fey-style show about a show. But if it is hard to see where their real lives stop and their on-screen characters start, that is because McGee is an uncommonly observant writer. From the Friday-night chip shop order to the minutiae of laundry day, Derry Girls revels in everyday detail to a Seinfeldian degree. When you stir in confused teens, perpetually irritated parents (“You can’t ring Childline every time your mother threatens to kill you!”), and McGee’s extraordinary dialogue, the result is TV dynamite. After the first episode, one reviewer commented that there wasn’t much of a storyline. Which, if you’re looking for weapons-grade narrative, is true: not much ever happens. But it’s very, very funny.

The political situation of 90s Northern Ireland is largely in the background, with armed soldiers caught in shot as the Steadicam glides past – though sometimes it is a peg on which to hang jokes. When English troops with machine guns board the school bus at a checkpoint, James freaks out. “What’s going on?” he whispers anxiously. “Dunno,” says Michelle, louder, nodding at one of them. “But do you think if I told him I had an incendiary device down my knickers, he’d have a look?”

There are no great quests, for love or even a meaning to life. Instead, the viewer is treated to a litany of abortive journeys and excellent putdowns (“Ach, Fionnula what about ye?” Erin’s Aunt Sarah says, on finding the owner of the chip shop in the living room. “I thought I could smell vinegar.”) Then there are the inconsequential crushes and dismissive nuns. Headmistress Sister Michael, an audience favourite played by Siobhán McSweeney, is a combination of all the teachers McGee had as a pupil.

“I don’t know if they make them like that any more,” she says. “That type of female teacher, who could be just thinking about looking at you and you’d be shaking.” When she announces upcoming GCSEs, Sister Michael says: “I know how daunting resit examinations can be, so if anyone is feeling anxious or worried, or even if you just want to chat, please, please, do not come crying to me.” Crucially, in Derry Girls, no lessons are ever learned.

‘I don’t know if they make them like that any more’ … Siobhán McSweeney as Sister Michael in Derry Girls. Photograph: Channel 4

There is no shortage, these days, of shows about schoolchildren and their family lives, but Derry Girls stands out. McGee says she was heavily influenced by My So-Called Life, the series about Pittsburgh teenagers taking on everything from alcoholism to child abuse. The show, which aired in the mid-90s, centred on the character of Angela Chase, played by Claire Danes.

“She had thoughts and opinions that, until I watched the show, I’d assumed were unique to me,” says McGee. There were, however, a few things McGee was not quite so able to connect with, in particular the fact that Chase was effortlessly beautiful and got off with Jared Leto. Such disconnects feed into Derry Girls, giving the show its two big themes: the irrelevance of cool and the unimportance of male validation.

Jamie-Lee O’Donnell, who plays gobby lad-mad Michelle, thinks back to her own all-girl Catholic school. “There wasn’t a cool crowd and a geeky crowd,” she says. “There were different characters, but everyone was on a level playing field.”

Orla, a Renault Clio obsessive who is Derry’s answer to Phoebe from Friends, certainly isn’t one of the cool crowd, but the character is a favourite not only with viewers but also with most of the actors’ parents, which may be just a little disloyal of them. (“It’s so weird that they tell us!” says Coughlan.)

No lessons are ever learned … the Derry Girls cast in series two. Photograph: Peter Marley/Channel 4

Louisa Harland, who plays Orla, wishes she could be more like the baldly indifferent teenager in real life. “I kind of based my portrayal on Tigger,” she says, referring to Winnie-the-Pooh’s energetic friend. “He’s a flibbertigibbet, a will-o’-the-wisp, a clown.” She seems to thinks that’s a line from AA Milne’s tales but it’s actually from The Sound of Music, a mix-up that makes her more like Orla than she realises.

For all its realism, Derry Girls is played – with impeccable timing – for laughs not tears, which makes the show a young female actor’s dream. Saoirse-Monica Jackson, who portrays Erin, the poetry-writing ringleader with delusions of grandeur, says: “To have the opportunity to do this scale of physical TV comedy – you don’t often see that for young women.”

Coughlan agrees: “You can tell there’s no male gaze, for how ridiculous we look a lot of the time. I love working with women who are not afraid to look … not ugly, but we do contort our faces a lot.”

While naturally circumspect on what’s in store this season, McGee says the political landscape shifts dramatically, but otherwise it’s very much your typical friendship stuff: James tries and fails to stand up for himself, Erin listens a couple of times to Orla, the girls fall out and in, the lot of them still and for ever “the eejits taking on the world”.

In the second episode of series one, they filmed the gang strutting down the street to the Reservoir Dogs soundtrack, lugging a jobs noticeboard they stole from the chip shop. (“Couldn’t you have just removed the ads?” someone asks. “They’re only Blu-tacked on!”) It was at this point that McGee realised she had found the losers she’d been searching for. She turned to Michael Lennox, the director, and gasped: “There they are!” As she says now: “They just looked like such a bunch of lovable dicks. I thought, ‘People might get this.’”

McGee has a particular overarching political timeframe in mind for the show, with the cast remaining around the same age. Which gives Derry Girls something of a narrow window since, even a year from now, theirs will be less of a gang of girls and more a gang of young women (Coughlan is already 32, mind, but still).

McGee is not, however, averse to the idea of a movie, though she does cite the 90-minute length of a typical film as her primary stumbling block. “That terrifies me,” she says. “But I’d love to give it a crack.”

• Derry Girls returns on Tuesday 5 March at 9.15pm on Channel 4.