O'Neill moved to New York City in 2010. Upon returning to Ireland in 2011, O'Neill began her first novel Only Ever Yours, which was published in 2014. She has since won the Sunday Independent Newcomer of the Year at the 2014 Bord Gáis Energy Irish Book Awards; the Children's Books Ireland Eilís Dillon Award for a First Children's Book; and The Bookseller's inaugural YA Book Prize 2015. The success of her debut, originally published as a novel for Young Adults, led Quercus to issue a new edition in 2015 aimed at a general audience. The Guardian called O'Neill "the best YA fiction writer alive today".
Her second book, Asking For It, was a number-one bestseller in Ireland and won numerous awards, including being named Irish Times Book of the Month in September 2015, Book of the Year at the Irish Book Awards 2015, the honour prize for fiction at the CBI awards 2016. and the American Library Association's Michael L. Printz Honor for excellence in literature written for young adults. The New York Times called it "riveting and essential". Asking For It was one of the top ten best-selling books in Ireland in 2016.
O'Neill works as a freelance journalist for a number of Irish national newspapers and magazines, covering feminist issues, fashion and pop culture. As of 2016, she has written as a weekly columnist for the Irish Examiner. She was a contributor to I Call Myself A Feminist, a collection of essays from women under 30 explaining why they see themselves as feminists. She won the Literature Award at the Irish Tatler Women of the Year Awards 2015, Best Author at the Stellar Shine Awards 2015 and the Praeses Elite award by Trinity College Dublin.
She hosted the RTE2 documentary, Asking For It?: Reality Bites, based on her second book, which aired on 1 November 2016. In this documentary O'Neill explores the issue of consent and tackling sexual assault and rape culture in Ireland.
Asking For It has been adapted for stage by Landmark Productions, premiering at the Cork Midsummer Festival 2018. It then went on to a sold-out run at Dublin's Abbey Theatre.
O’Neill's third novel Almost Love was published in March 2018 by Riverrun. The Surface Breaks, a reimagining of The Little Mermaid, was published by Scholastic in May 2018.
- Only Ever Yours (2014)
- Asking For It (2015)
- Almost Love (2018)
- The Surface Breaks (2018)
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