CONOR McPherson's The Weir, a play about people talking in a rural pub in the northwest of the Irish Republic, is one of the most successful Irish plays of the last 20 years. A night of conversation between four local men (publican Brendan and three local barflies) and Valerie (a Dublin woman, recently moved into the area), it seems an unlikely blockbuster.

Yet the drama has met with critical and audience acclaim ever since it premiered at the Royal Court theatre in London in 1997. On the face of it, the play is merely the telling of tall tales, but, as Amanda Gaughan, director of the new production at Edinburgh's Royal Lyceum, observes, there's something special going on in the writing.

Having never seen the play on stage, Gaughan read it as one of a number under consideration for the Lyceum's 50th anniversary season. She was looking, she explains, for a play with "visceral" impact.

"I totally had that gut reaction to it," she remembers. "I got to the end of the play and was asking myself, 'How did I get here?' It's just so simple. It's beautifully written and the characters are so recognisable."

Anyone who knows The Weir will recognise Gaughan's sense of the play insinuating its way, almost by stealth, into one's psyche and emotions. Critics often describe the piece as "unassuming" or "modest", while acknowledging its great psychological and emotional impact.

"It is just people talking and telling stories, it's very Celtic that way," says Gaughan. "You think it's just going to be ghost stories, but, actually, there's a real generosity and humanity in the play."

Gaughan, herself a Scot with family roots in Donegal, uses the word "Celtic" precisely. Scotland is, she believes, a perfect country in which to stage McPherson's play.

"We [the Scots and the Irish] all like a good blether," she laughs. "That's one thing that we share. We do like to tell stories."

There's no doubting McPherson's capacity for storytelling, and for crafting characters who know how to tell them. However, there's more to the success of the play than that.

The Weir is a drama of beautiful structure and rhythm; there is even an academic paper, by Kevin Kerrane, entitled The Structural Elegance Of Conor McPherson's The Weir. Elegance is right. If the actors and director have captured the rhythm of the writing, the play will take you along gently, like a boat on a calm sea.

"In rehearsals we know when we don't hit the rhythm right", Gaughan agrees. "You know when you've dropped the ball, because you can feel it."

The culmination of the play's writing (which is simultaneously comic, contemplative, muscular and poetic) and its elegant, rhythmical structure is a paradox. It's a big stage play with the intimacy of a small pub in rural Ireland.

"The play does creep up on you," says the director. "It's almost subliminal, the way it does that.

"We're trying to capture something of that in the stage, lighting and sound design. The design team is working both to bring out the epicness of the play and to make people who are sitting in the upper circle feel like they are in someone's living room.

"In the play, Jack [the local mechanic] says, 'It's been a strange little evening for me.' If we do the production right, members of the audience should feel that way, too."

The Weir runs at the Royal Lyceum, Edinburgh until February 6. For more information, visit: lyceum.org.uk