McNulty's stage experience began with ballet and amateur performances at
An Grianán's "Youth Theatre" in
Letterkenny. In 2014, McNulty co-starred in the
RTÉ One series
Clean Break as inquisitive child Jenny Rane. In 2015, McNulty appeared in
Agatha Raisin as the younger version of the titular character and guest-starred in
CBBC's
The Sparticle Mystery. McNulty made her film debut in
sci-fi thriller
Morgan, portraying the 10-year-old iteration of the titular character. From 2017 to 2019, she starred as
Anne Shirley-Cuthbert in the
CBC and
Netflix drama series
Anne with an E, an adaptation of the 1908 novel series
Anne of Green Gables by
Lucy Maud Montgomery. McNulty was cast from a selection of 1,800 girls for her ability to deliver dialogue which is "incredibly thick and dynamic and beautiful"; "her audition consisted of talking to trees, chatting with flowers and building thrones out of twigs." McNulty received critical acclaim for her portrayal; Gwen Inhat of
The A.V. Club lauded her "absolute possession" of Anne as well as her ability to make "fanciful language sing", while
Neil Genzlinger from
The New York Times wrote McNulty's performance was "wonderfully ebullient and eminently likable". Her portrayal earned her the
Canadian Screen Award for Best Television Actress and the
ACTRA Award for Best Performance – Female. McNulty portrayed the lead role in the 2024 indie thriller
She Came Back (pre-release title
Maternal), the feature directorial debut of actress
Megan Follows, which was filmed in early 2020. She also starred in
Black Medicine (2020), in which she portrays an alcoholic and drug-addicted Irish teenager. In December 2020, McNulty was cast in director
Michael McGowan's film adaptation of
All My Puny Sorrows, a novel by Canadian author
Miriam Toews. In June 2021, McNulty was cast in the
fourth series of the
Netflix drama
Stranger Things, portraying Vickie, a "cool, fast-talking band nerd". She reprised the role for its
fifth and final series. ==Personal life==