Ms. Rooney, who wrote “Conversations With Friends” while studying for a master’s degree in American literature, expected to reach readers like herself, “people who share my ideology or have a similarly jaundiced view of social systems.” Her mass-market success is clearly still a little disorienting. “Light and sparkling is the phrase that has been used,” she said. “I can’t complain if people think it’s sparkling, but then there’s a sense that wasn’t what I set out to do.”

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As a student in a Catholic high school a decade ago, Ms. Rooney was required to attend lectures by Pure in Heart, an organization that discouraged premarital sex. The presenters, having gathered a roomful of teenage girls, would ask for a volunteer to extend her arm and would display a length of clear adhesive tape, telling them that the tape signified them as virgins.

“This is you when you decide to sleep with your boyfriend,” they would say, attaching the tape to the girl’s arm and peeling it off, now cloudy with skin cells. Then they would do the same thing multiple times, to signify multiple partners, so that the tape was clogged with dirty particles, and hold it up before the class, asking, “Would you want to marry this?”

Ms. Rooney and her classmates sat there, smirking. “No piece of Sellotape strikes me as an adequate marital partner,” she said. “We perceived them as bizarre.” The abortion ban, she said, was more offensive: “It felt like a vestige of a culture that was not in tune with how people were living their lives.”

Ms. Rooney’s fictional twentysomethings furnish a kind of response. In the Dublin circles she describes, the Roman Catholic Church barely figures. Relationships are everything. Sex is described with great care and detail. (“I could hear myself making a lot of noise, but only syllables, no real words. I closed my eyes. The inside of my body was hot like oil,” says Frances, the narrator of “Conversations With Friends.”) She writes attentively of pain, offering eye-watering descriptions of menstrual cramps caused by endometriosis.

Ms. Rooney arrived at Trinity College, Ireland’s most elite university, from Castlebar in County Mayo, where her father worked as a technician for the state-owned telecom company. Her parents were socialists; they so often repeated Marx’s slogan “from each according to their ability, to each according to their needs,” that as a child she took it to be “a religious quote, or maybe a parenting guideline.” She found her tribe as a competitive debater — at 22, she was the top debater in Europe — and settled into what she would later describe as “an analytic way of living.” She and her friend Aoife daydreamed about being “a brain in a jar,” liberated from the encumbrance of a body.