Anna Burns has become the first Northern Irish author to win the Man Booker prize, taking the £50,000 award for Milkman, her timely, Troubles-set novel about a young woman being sexually harassed by a powerful man.

The experimental novel, Burns’s third, is narrated by an unnamed 18-year-old girl, known as “middle sister”, who is being pursued by a much older paramilitary figure, the milkman. It is “incredibly original”, according to the Booker’s chair of judges, the philosopher Kwame Anthony Appiah.

“None of us has ever read anything like this before,” said Appiah, announcing the win at a dinner at the Guildhall in London. “Anna Burns’s utterly distinctive voice challenges conventional thinking and form in surprising and immersive prose. It is a story of brutality, sexual encroachment and resistance threaded with mordant humour.”

Written with few paragraph breaks, eschewing character names for descriptions, Appiah admitted that Milkman could be seen as “challenging, but in the way a walk up Snowdon is challenging. It is definitely worth it because the view is terrific when you get to the top,” he said. “I spend my time reading articles in the Journal of Philosophy so by my standards this is not too hard. And it is enormously rewarding if you persist with it. Because of the flow of the language and the fact some of the language is unfamiliar, it is not a light read [but] I think it is going to last.”

As the award was announced, Burns was lost for words. At the press conference afterwards, the 56-year-old author said her job as a novelist was “to show up and be present and attend. It’s a waiting process.” She “just had to wait for my characters to tell me their stories”.

Asked about how she had filled the long gap since her 2002 Orange prize listing for No Bones, she said she had done commercial events and moved houses. What will she do with the money? “I’ll clear my debts and live on what’s left.”

The win makes Burns the first Northern Irish winner – previous Irish winners, including John Banville, Anne Enright and Roddy Doyle, all come from the republic. It also makes her the first female winner since 2013, when Eleanor Catton took the award with The Luminaries.

Burns beat writers including the American literary heavyweight Richard Powers, Daisy Johnson, at 27 the youngest author ever to be shortlisted for the award, and the Canadian writer Esi Edugyan. According to Appiah, the judges, picking from a shortlist that delved into some dark themes, were “unanimous” in their choice of winner – and not influenced by concerns that picking a third American winner in a row could cause controversy.

Burns, who was born in Belfast and now lives in East Sussex, drew on her own experiences growing up in what she called “a place that was rife with violence, distrust and paranoia”. As the milkman presses his advances on the reluctant middle sister, rumours begin that she is having an affair with him. “But I had not been having an affair with the milkman. I did not like the milkman and had been frightened and confused by his pursuing and attempting an affair with me,” Burns’s narrator tells us.

“I don’t know whose milkman he was. He wasn’t our milkman. I don’t think he was anybody’s. He didn’t take milk orders. There was no milk about him. He didn’t ever deliver milk.”

Appiah said: “This woman living in a divided society is harassed by a man who is sexually interested in her. He is taking advantage of the divisions in society to use the power he has because of the divisions, to go after her. Sectarianism and divisions in Ireland play an enormous role in the novel [but] Northern Ireland is not the only place in the world with a divided society … TS Eliot said you can’t be universal without being particular, and this is particular but brilliantly universal as well.”

Milkman also spoke to the concerns of today, he said. “I think this novel will help people think about #MeToo ... It is to be commended for giving us a deep and subtle and morally and intellectually challenging picture of what #MeToo is about.”

Appiah added that Burns’s narrator, a girl who reads as she walks (“This would be a 19th-century book because I did not like the 20th century”), has an “extraordinary” voice.

“She is sustained by her own good sense, by her humour and by her reading. She is a literary person in a rather unliterary society,” said Appiah, who was joined on the judging panel by the crime writer Val McDermid, the critic Leo Robson, the feminist writer Jacqueline Rose, and the artist and graphic novelist Leanne Shapton.

Burns revealed in the Guardian last week that Milkman has its origins in “a few hundred words that were superfluous in a novel I was currently writing”. She tried to craft a short story from them, and they turned into Milkman.

“The book didn’t work with names. It lost power and atmosphere and turned into a lesser – or perhaps just a different – book. In the early days I tried out names a few times, but the book wouldn’t stand for it. The narrative would become heavy and lifeless and refuse to move on until I took them out again. Sometimes the book threw them out itself,” she said.

• This article was corrected on 17 October 2018. The last female winner of the prize was Eleanor Catton in 2013. It was not Hilary Mantel, nor was it in 2012 as originally stated.