Marina Carr's new play was delivered in a Dublin maternity ward. Georgina Brown talks to the author about her heroine's mixed parentage - part Beckett, part Bog of Allen

Chronologically confused, determinedly non-narrative, written in dialect, Marina Carr's latest play does not sound like everyone's idea of a grand night out, but it has proved precisely that in Dublin. "Breathtaking", "stunning", wrote the critics, exhilarated by its audacity and intensity. Following its Londo run at the Royal Court, Garry Hynes's production of Portia Coughlan will return to the Abbey Theatre for an extended season.

There are many to thank for this remarkable play, first the great and good who decided that the centenary of Dublin's National Maternity Hospital should be celebrated with an extensive arts programme, second the 80-odd women (among them Fiona Shaw, Maeve Binchy, Brenda Fricker and Garry Hynes) who stumped up pounds 50 to commission Marina Carr to write a play.

Every day she walked past wards of mothers and babies to get to the room the hospital provided for her to write in peace. "You pick things up from an atmosphere and sure it made a difference - though you never know where writing comes from," says Carr. She had a year to write the play but completed the first draft "in a white heat" in five weeks. "In the olden days they used to think of writing as more like witchcraft and there's something in that."

The landscape of the play is literally and metaphorically boggy and unstable, with water, water everywhere. The setting is the Irish Midlands where Carr grew up, "a very beautiful place, full of lakes and rivers and mountains and surrounded by the Bog of Allen". The words themselves seem waterlogged, the dialect long on vowels and short on consonants, and further slurred and blurred by drunkenness and grief. It's a heightened, poetic and theatrical language, "based on the way English is spoken in the Midlands - though if I wrote it like it really is, nobody would understand it. It's long and slow and flat and every second word is a curse," says Carr, whose mother, a schoolteacher, sent her brood of six to elocution lessons "to get the bog out of them". The Carrs were "blow ins", outsiders, and Carr suspects that this accounts for her objective perspective on the place. The family kept a sweet-shop and neighbours were treated to their homegrown theatricals, written, directed and performed by the Carr children in their garage. Only then were they allowed to buy their sweets.

This, her sixth play, concerns clever, crazy, self-destructive Portia, 30 years old today and inextricably bound to her twin brother, Gabriel, who drowned himself 15 years before on their 15th birthday. She is drawn irresistibly to the river Belmont where he died. Her twin, the murky depths of the river, the past, all conspire to claim her. And they do. In the second act we see Portia dead. But the third act returns to the (in)action on that nightmarish 30th birthday, our response altered by our knowledge of what will happen. Or thinking we know. The director, Garry Hynes, refuses to commit herself: "I can't say it is or it isn't. The audience sees her dead and draws its own conclusions and the fact that you mourn the death of a character in the middle has to make a difference."

Not that the story is anything so crude as being a journey to death. "I had the womb all to myself," laughs Carr. "But everyone is aware of possibilities crushed or lost - and the side that is cut off is often the most beautiful side."

Her first serious play, Ulalloo (an old Irish word meaning death-song), was written in her final year at university in Dublin and was influenced by her immersion in Beckett. It was immediately accepted by the Peacock Theatre, which has kept a sharp eye on Carr's work.

Portia Coughlan is a departure. The action is stretched over four generations of a family, as a destiny set in motion in the distant past determinedly works its way into the present, and new literary influences seem evident. The tragedy is Greek; the names Portia and Belmont bring the resonance of Shakespeare; the mood is Tennessee Williams and Eugene O'Neill - despite some humour, it is exceptionally bleak. "I'm not so bleak myself," says Carr. "I'd not be alive if I was." Today she's high as a kite, having just seen the production of Six Characters in Search of an Author at the Abbey. "I was blown away. I hadn't registered the real craft before. Nothing sluggish or sequential, yet it obviously obeys the rules of sequence." You could say the same about Portia Coughlan, where the sequence is an emotional line, pulled taut to breaking point with dazzling skill.

n 'Portia Coughlan' opens tomorrow at the Royal Court, London SWI, and runs to 1 June. Booking: 0171-730 1745