In Ireland next week, the new show from Forced Entertainment takes the legendary company's staged chaos to a new level, its leader tells Peter Crawley.

You are what you eat at the Maison Bertaux, a Soho patisserie incongruously situated between strip clubs, porn shops and a theatre advertising a forthcoming Mary Poppins musical. Here Tim Etchells is known not as the director, text generator and occasional performer of Forced Entertainment, Sheffield's legendary experimental theatre company, but simply as "balding; almond croissant".

It is the morning after the first sell-out London performance of Bloody Mess, the company's 20th-anniversary show, which is coming to Ireland next week as part of Project arts centre's About Time festival, and Etchells cautiously greets his interviewer ("café au lait; pain au chocolat") as a video installation, entitled Dissidence And Cake, flits across the café walls. "All cake is a sly encouragement to test the edges," it tells us. I ask how it feels to be hip at 20.

"We're a group of people who, since the beginning, have been furrowing our own field," he says with a voice that seems to trail through gravel, "always walking our own road in terms of the kind of work that we do." That work began in 1984 with the story of a cinema usherette whose life blurs into the films she watches. It has progressed to works for which the text is improvised by the company, then continually remoulded and reordered by Etchells, and which have included game sequences, such as cast members re-enacting their favourite movie deaths, using ketchup for blood.

It has moved, with near inevitability, to "durational performances" lasting anything from six to 24 hours, of signature pieces that are almost wholly improvised but guided by easily comprehensible rules, making them both chaotic and compulsively involving, works that bristle with, as a character from Bloody Mess might put it, potentiality. The Guardian has called them Britain's most brilliant experimental theatre company, an opinion that seems to hold greater sway outside of Britain than it does inside it.

"I think we always knew that theatre culture as a whole in England is very conservative," says Etchells, "and that the only way to prosper would be to take our time and do what we want to do. We've found a lot of support in mainland Europe and elsewhere, which has really been, at times, the only thing that's made it possible for us to continue to work. It's taken so long, really, for us to find a place, although I still think there are battles to be fought."

There was a period in the 1990s, for example, when Forced Entertainment was at risk of losing funding from the English Arts Council for supposedly poor production values. "There is in our work a very precisely, pointed trashiness and a kind of rawness and thrown-togetherness which we actually worked on very carefully," he says. Now, however, they have been given "a corner to play in", as Etchells puts it, sardonically. "If we don't bother anybody it's fine."

If the London audience for Bloody Mess is anything to go by - the vast majority of whom must have been barely toddling when the company first began its approach to performance, technology and the fractured narratives of cities - nobody puts Forced Entertainment in the corner. A calamitous mixture of clowning, confessions, stand-up, animal costumes, stage combat, rock-concert roadies, gratuitous nudity, lessons in crying and ad-hoc lectures on the big-bang theory, the production honours every critical dismissal of the company by producing, literally, a bloody mess. But to hear Etchells explain the meticulous process underpinning it, well, this is another fine mess he's gotten us into.

"This show, in a way, has the kind of structure that early pieces of ours have explored. It was devised by having more than several figures on stage who are inhabiting quite separate worlds - worlds that are sliding past each other, and colliding, and the sparks are flying. It can be a really difficult ride if you're expecting a well-made play, but it's not out to pick a fight with you. It's out to have a good time with you."

Initially, with any Forced Entertainment piece, the company (there are six permanent members) will improvise with different combinations of costume, text, image, props and actions before finding something "interesting". The process is meticulously documented on video. There follows a lengthy process of transcribing, choreographing or scoring the improvisations, then rehearsing them, often switching roles and moving sequences from one place to another on the time line, of "seeing what kind of architecture you can create".

It is a crashing together of chance and order that, Etchells agrees with a chuckle, is almost perverse, leading the company architect to determine the givens of the performance while devising. "You know you've got these clowns that hate each other, that Claire [ Marshall]'s going to be in a gorilla costume, probably talking about sex whenever she takes her gorilla head off." It is not, he insists, a process that comes easily. "I don't think five years ago we would have been able to contemplate organising or dealing with a mess as spectacular as this one."

In Certain Fragments, a book he wrote in 1999, Etchells describes his theatre as understandable to "anybody brought up in a house with the television on", where narratives are simultaneous, competing, unresolved. Such fragmentation is his ideal, he says, a fragment being "both statement and question".

At times such entertainment can seem a little too forced. Bloody Mess often threatens to cohere, to allow points or scenes to flow into each other, and often directorial intervention seems necessary to prevent the fragments from uniting. "I think that as a watcher," says Etchells, "I like to keep things open and keep a sense of ambiguity. I don't like things to confirm themselves too much as one thing. So, yes, a lot of my work as a director or a structurer is to stop things from, I would say, from collapsing." By collapsing he means making sense - or, perhaps, making just one kind of sense.

"I'm a great truster in and believer in the spectator, the audience. I don't like to be told things. In plays where, two-thirds of the way through, characters suddenly start talking in great big long speeches about one thing or another, you sense the playwright hovering. In everything - a film or theatre or a novel - the moment when I'm being told something is when I'm leaning away, thinking, oh, please."

He drops his head into his hands. "You've now collapsed the integrity of the thing you were working on, because you've popped up in the middle to tell me something." He picks himself back up. "I like to leave things for people to find. The joy of this show especially is of the roving eye: you can always wander off the main thing and find somebody else who is doing something stupid or amusing or disturbing. To me, realising your own agency - your own significance in terms of making meaning - to me that's politics. That's saying you make the difference, you're the person putting this together, you join the dots. . . .

"Most of us were interested in film and music and other things, and theatre was this curious object that we picked up in '84, and we're still kind of scrabbling at it, trying to figure out if you can break it . . . and also what happens if you do. What can you make once you've broken it?"

He makes them sound like theatre vandals. "I like that idea," he smiles. "We've grown to love and respect the form we're in, but it's also a relationship that's quite idiotic and vandalistic: take a hammer to it. If it won't work, bang on the top of it and see what you can get it to do."

Bloody Mess is at Project, Dublin, next Friday and Saturday, as part of the About Time festival. Tim Etchells is giving a talk at Project on Saturday, November 27th at 2pm.