The “glittering intelligence” of Sally Rooney’s story of the affair between a student and an older actor has won the 26-year-old Irish debut novelist the Sunday Times/Peters Fraser & Dunlop Young Writer of the Year award.

Rooney’s Conversations With Friends is narrated by Frances, a student in Dublin, who along with her fellow student Bobbi is befriended by the journalist Melissa and her husband Nick, an actor. As the relationships between all four change and deepen, Frances, who is 21, has an affair with Nick.

Speaking to the Guardian earlier this year, Rooney described how she wrote the novel at “huge speed” in three months, working 16 to 18 hours a day. “Dialogue is the most fun to write,” she said. “It’s kind of like a tennis match … it has to go back and forth.”

Conversations With Friends beat titles by the authors Minoo Dinshaw, Claire North, Julianne Pachico and Sara Taylor to the £5,000 prize, which goes to the best work of fiction, non-fiction or poetry by a British or Irish author aged between 18 and 35, and has been won in the past by Zadie Smith, Ross Raisin and Sarah Waters.

Judge and Sunday Times literary editor Andrew Holgate said that: “for line-by-line quality, emotional complexity, sly sophistication and sheer brio and enjoyment, Sally Rooney’s Conversations With Friends really stood out.”

“To have produced a novel which nods all the way back to Jane Austen’s Emma, while being so thoroughly modern in feel, is quite something, and Rooney proves herself with this debut to be a really worthy addition to the extraordinary list of past winners of the Young Writer award,” Holgate added.

Fellow judge Lucy Hughes-Hallett said Conversations With Friends “stood out for its glittering intelligence, its formal elegance and its capacity to grip the reader”, and praised “the sophistication of its erotic quadrille”.