Lynch was born in Termonfeckin, County Louth, Ireland, the daughter of Marguerite and Donal Lynch. She has two older sisters, Emily and Mairead, and one younger brother named Patrick. As a child, Lynch read the Harry Potter series and became a fan, sending letters to the author, J. K. Rowling. She attended Cartown National School in Termonfeckin until June 2004 and then moved to Our Lady's College in Drogheda, where her father was the deputy principal. In 2008, Lynch studied speculative fiction and drama at the Centre for the Talented Youth of Ireland, a summer school for gifted teens, in Glasnevin. While on the Harry Potter set, she was tutored for at least three hours a day. From September 2010, Lynch attended the Institute of Education to repeat her Leaving Certificate.