It's been six years since Nicola Coughlan stepped on to the set of Derry Girls for the very first time.

A journey the actress describes as nothing short of "mad", it's a period that will forever be immortalised by plaid school uniforms, elaborate dance routines and the outspoken views of a particularly vocal Catholic nun.

But as audiences ready themselves for the third and final series of the Bafta-nominated sitcom, its cast are pensive. With the gates of Our Lady Immaculate College poised to close on their characters for the final time (or at least on television, given creator Lisa McGee's big screen ambitions), breakout star Coughlan and co-star Jamie-Lee O'Donnell have already begun lamenting the loss of their on-screen companions.

"It's a weird thing, grief, saying goodbye to a character," reflects Coughlan, 35, following a lengthy pause. "It's a really strange thing that's very hard to explain to people. I mean, Clare Devlin is a mess, but I love her and I will miss her very much," she adds, referencing her academically ambitious character.

"I don't know if I'll ever be able to fully say goodbye in my head," agrees co-star O'Donnell, 30, who plays outspoken Michelle Mallon in the series.

Set against the backdrop of Northern Ireland's political conflict, Derry Girls is the coming-of-age tale audiences never knew they needed. Depicting the heart-warming escapades of four girlfriends and a "wee English fella" as they navigate the awkwardness of adolescence, the first nostalgia-packed series had viewers pining for the 90s. Now, four years on from its television debut, the series is ready to tie up the remaining loose ends as the teens anxiously await their GCSE results.

"We always knew it was going to end at the time of the Good Friday Agreement, that's something Lisa's talked about quite a lot," says Coughlan.

Recounting how she received the final scripts for Derry Girls mid-way through shooting the "big, heavy stuff" for Bridgerton's series two finale, the actress says she was "really stressed" at the thought of potential filming clashes. "I was like, 'I can't even look at these because my head's gonna explode!" adds Coughlan.

"I was so worried I wasn't gonna be able to film Derry Girls and everyone would hate me," says the star, looking visibly distressed over Zoom. "We did a night shoot on the Thursday for Bridgerton, which wrapped at 5am on Friday morning. I went home, packed, flew on the Saturday, on the Sunday I had a costume and wig fitting, and then was filming Derry Girls on Monday."

Swiftly transforming into something of a global phenomenon, the debut series of Derry Girls became Channel 4's biggest comedy launch in over 14 years. The popularity led to Netflix subsequently acquiring the rights, attracting a new legion of fans on the far side of the Atlantic.

Describing the show's popularity in the US as "super crazy", Coughlan admits she initially questioned its potential to attract audiences beyond the Irish border, given the specificity of its subject matter. Little did she know the cultural cornerstone the show would become.

"When Tarantino was in The Simpsons, he said he knew he'd really made it. So we're up there now - I'm practically Tarantino in my mind," laughs co-star O'Donnell, referencing the legendary Pulp Fiction director. A nod to a recent episode of the US animated comedy which paid tribute to the show by naming its ice-cream parlour 'Dairy Girls', the honourable reference saw series creator McGee take to Twitter and dramatically proclaim: 'I. Am. dead.'

In spite of the recent international attention the show has garnered - bringing with it the temptation to further extend its television run, McGee has stuck to her guns. Series three was always the finishing line. The writing talent behind shows including The Deceived and Being Human, she's continued to nurture the show's characters as though they were her very own children.

Noting the way in which the creator would regularly check-in with the cast in order to reassure them about the fate of their characters, O'Donnell, who most recently starred in Channel 4 prison drama Screw, says the beauty of McGee's writing lies in its "warmth".

"I just thought it was such a beautiful way to look at it - that as the writer and creator of us, as she was writing the last season, she was always conscious that we were going to be okay," says O'Donnell, emotion clear in her voice. "She was organising in her head, our characters being okay after season three - like they were really people to her."

Exposing an altogether more vulnerable side to the usually fiery Michelle this series, O'Donnell recounts her innate desire to "protect" her on-screen character at all costs. "It sounds insane, I know that, but the thought of people watching Michelle when she's not, quote unquote on form, makes me feel a wee bit protective over her," adds the actress. "It made me want to cry a wee bit."

Describing the show as a "love letter" to McGee's birth city of Londonderry, fellow actress Saoirse-Monica Jackson, who plays the highly strung Erin Quinn, notes the sense of "honesty", "authenticity" and "innocence" captured in McGee's writing.

"I think it was the first time these sort of stories were told in a show from the North, that wasn't in such a dark light," says Jackson, 28, with a nod.

"She writes with such warmth - and I think that really connected with everybody. It was everybody's baby. We all really held the sentiment that it was all of our show. And I think everybody really put their best foot forward and that made for magic."

Adding: "I sound like a fridge magnet."

Derry Girls returns to Channel 4 and All4 on Tuesday, 9.15pm.