Born in County Louth, Ireland, Lynch made her film debut in Harry Potter and the Order of the Phoenix (2007) and reprised her role in successive sequels to critical praise, concluding with Harry Potter and the Deathly Hallows – Part 2 (2011). Lynch appeared in G.B.F. (2013), which premiered at the Tribeca Film Festival to positive reviews. She made her stage debut in Houdini as Bess Houdini, which toured the UK in 2013. Lynch starred in the indie drama My Name Is Emily, which premiered at the 2015 Galway Film Fleadh to critical acclaim. In 2017, Lynch starred in revival of Disco Pigs at the Trafalgar Theatre in London. In 2018, she competed and scored third place on season 27 of Dancing with the Stars. She went on to star in the British stage adaptation of The Omission of the Family Coleman at the Theatre Royal, Bath in 2019.

As an activist, Lynch advocates for veganism and animal rights. She has been involved with several non-profit organisations and launched both a vegan-themed podcast and the cruelty-free cosmetics brand Kinder Beauty Box.