Marie Mullen is returning to Druid, the group she founded, for the first time since 1996. How is it to be back, asks Belinda McKeon.

On a rainy afternoon in Galway the small lobby of Druid Lane Theatre seems even more cramped than usual. Lining it are thick bunches of broken branches and piles of what looks like firewood; up against the back wall stands an old-fashioned dresser. In among other antiquated pieces of furniture the actor Diarmuid de Faoite is whispering to himself the lines of a frenzied priest; beside him sits Norma Sheahan, keeping to herself for now the voice of a young woman who wants desperately to be a bride.

The rehearsal schedule for the day is tacked up over the long table of teacups and biscuits; making their way here by now, for an afternoon call, must be the would-be groom, elderly smith, wandering friar and fine-looking girl with fair hair. Put together, they comprise almost the entire dramatis personae of The Well Of The Saints and The Tinker's Wedding, two plays from the J. M. Synge canon, lesser known than The Playboy Of The Western World but combining, with similar vivacity, sparks of savage irony with scenes of surprising compassion. And, as did Playboy earlier this year, they are about to receive new productions by Druid as part of artistic director Garry Hynes's DruidSynge, the staging of every one of Synge's six plays over 18 months.

From tonight Well and Tinker's, as the actors call them, will appear together on the Druid Lane stage before touring to Dublin, Ennis, Letterkenny and the plays' spiritual home, in Glendalough, Co Wicklow. Mounting productions simultaneously is a task at which Druid has proved itself more than capable, given the success of Martin McDonagh's Leenane trilogy in the mid-1990s. But the nervous excitement this afternoon suggests the challenge grows no smaller for having once been conquered. When the doors to the rehearsal space are pushed open, spilling out three of the chief protagonists of this latest undertaking, and of the company itself, the white noise of creative energy reaches full pitch.

Hynes comes first, consulting the production staff about the gathering of props. After her, managing to look at once exhausted and delighted, is Mick Lally, cast as Well's Martin Doul, the blind man who has his sight restored. Behind him is the actor playing both Martin's similarly cured wife, Mary Doul, and, in Tinker's, Mary the mother reluctant to let her son marry, fellow Druid veteran Marie Mullen. "Do I have a minute for a cigarette?" she asks; with just an hour for both this interview and her lunch break, she knows time is short. Not two minutes later she's back, stepping around bundles of branches in the upstairs rehearsal room. "A drag is all I needed, to settle myself," she says. "My head is mithered."

But Mullen's head isn't really mithered, as the phrase from her native Co Sligo would have it. Rather, it's mesmerised by the richness of the language Synge has given to his two Marys and passionately eager to hear it, to catch it, to deliver it as it was meant. To the audience the task of portraying two cranky, lonely, bitter Marys in the one night, and of making a distinction between them, might seem a significant feat. But to Mullen, it's clear, the challenge is not to double up but to tap in.

Having played Mary in The Tinker's Wedding before, she feels the challenge is particularly acute where The Well Of The Saints is concerned. "I wish I could describe this better for you," she says, and her gestures and facial expressions as she speaks betrays how deeply she believes in this. "He's written Well in a much different way than he wrote Playboy or Tinker's, or even In The Shadow Of The Glen, which I have also done. It is a fable, and so there is a certain formality and a certain density about the language, in that you need the rhythm as well as the meaning, and you need the whole lot, all at once, you see. So what it is, is trying to get the thoughts out with the words and not lose the rhythm, if you like."

She takes a deep breath. "So I'm finding it really dense and really hard to lift the language out, with the meaning, but it's very beautiful; and you want it, you want the language, and you want the emotion as well, because of course he does pack such an amount of emotion into every single thing he does. . . . I mean, he's the most extraordinary playwright I've ever met."

It's a slip of the tongue that couldn't be more apt. Mullen talks about Synge as if he were in the rehearsal room with them; for her he is a living presence, guiding them towards the realisation of these plays, giving them the lay of the land, as she describes it. Her search for the beat of his language may come across as abstract or overly technical, but it is bound up with a quest to connect with his characters, to uncover the trail laid by Synge, which she has known since playing Playboy's Peigin Mike in Druid's 1975 production.

"And I remember," she almost whispers, "not being able to handle the language, the length of the sentences, the verb coming at the end, thinking, what's that all about, and then suddenly, when you got it at the end, the flow. . ."

It sounds like learning Irish, like having the tuiseal ginideach hammered into you for months and years until, suddenly, it somehow clicks, and you're fluent. "That's right," says Mullen. "That's exactly, exactly, what happens with Synge. How . . . how did he know that?"

Somehow, Mullen believes, Synge infused the harsh landscapes of his plays - the "lonely, mountainous district" of Well and the dark "village roadside" of Tinker's - with an extraordinary poetry that, he knew, would be revealed to the voice of his readers and his actors. His is a language in which meaning emerges in whispers and shadows, in moods that emerge silently, gradually, until they are inescapable. The Douls' admission to themselves that the sighted world brings much harder truths than the blindness, the blackness in which they were happy; the loneliness of the priest in The Tinker's Wedding; the fear of the young woman, in that play, of the coming of old age and its associated detriment: all advance stealthily on the characters, twisting the directions they would take, harsh realisations that grow no gentler in the sharing. The truth, for Synge's characters, creeps in like the old age they dread.

For Mullen, his sense of the horror that the loss of physical beauty brings is one of the most striking aspects of Synge's work. "The lack of that beauty, for people, is so disastrous in Synge," she says. "And the way he turns that in on people's heads."

She recalls having to steel herself when she received the parts of two haggard women surrounded by young beauties, "having to make myself OK about being called constantly ugly, in the play I mean. Asking myself why did I take this part and put this on myself, every day".

Yet if youth and good looks are used as weapons by the younger women in Synge's plays - by Sarah Casey, so determined to marry Michael Byrne despite his mother's protests, and by Molly Byrne, whose pretty face becomes the pool of Martin Doul's hopes when his sight returns - still, pulsing through the language, emerging with the rhythm of his characters' words, is the sense that physical beauty is no truth but a deception better done without.

"It's like Mary Doul says," explains Mullen, "that, if they have their sight, what good would our grey hairs be itself, the way we'll see them falling each day and turning dirty in the rain. Whereas when they're blind they never see muck. They don't know what colour it is. And seeing lovely white hairs turning dirty in the muck. . . . When they don't see them, then they have them all the time. They treasure the white hair."

All of this from a woman whose own hair is a long way off whiteness. Like that of all of the founding figures of Druid, including her husband, Sean McGinley, Mullen's work garnered recognition at such an early stage - soon after the company's genesis, in the early 1970s - that it seems as if she has been on the stage for ever.

Mullen now lives in Dublin with McGinley and their two daughters, aged seven and 12; returning to Druid, to work with Hynes and Lally for the first time since Brian Friel's The Loves Of Cass McGuire, in 1996, has been, if not quite a homecoming - she hesitates when I use the term - then at least a return to something that she knows, somehow, innately. "There's a familiarity and a relaxed way. . . . You don't have to be polite or anything: we can tell each other, and we have a kind of shorthand. Which I don't understand, but people say that there is."

Returning to Galway, she sees many changes in a city that was merely a town when Druid started. "I remember my path into the theatre when I lived here 20 years ago being derelict," she says. But, she continues, in a brisker tone, "we must move on. And I suppose the theatre helps in that sense. Because it's so transient. You know, you do a play, you meet the people, you make friends, you have your thing and then it's finished. And then you're on to a whole different set of relationships. And yet you can come back to them and it's OK".

A lifestyle of changes makes a landscape of changes easier to accept, even love. Some changes, however, strike home with a reality that Synge's characters might well empathise with. "Everyone I knew here moved on," says Mullen. "Yet I see people now and I say, oh, I recognise that face. And then I realise that I'm seeing the daughter. The daughter, with the face of her mother that I knew when I was a young woman." And she laughs for a moment before heading back downstairs.

The Well Of The Saints and The Tinker's Wedding are at Druid Lane Theatre, Galway, from today until September 18th, before touring to Glendalough, Ennis, Dublin and Letterkenny