Kirby made her professional stage debut in a production of Arthur Miller's "All My Sons" at the Octagon Theatre Bolton. This marked the beginning of her career as a working actress.

Kirby gained early recognition for her performances in various stage productions, including appearances at the Royal Court Theatre, a renowned venue for new and innovative plays. This experience exposed her to challenging and thought-provoking material.

She played Isabella in Thomas Middleton's "Women Beware Women" directed by Marianne Elliott at the National Theatre.

She played Rosalind in Shakespeare's "As You Like It" at the West Yorkshire Playhouse.

Kirby starred as Stella Kowalski in a critically acclaimed production of "A Streetcar Named Desire" at the Young Vic, directed by Benedict Andrews. This performance garnered significant attention and established her as a rising star in the theatre world.

Starred in "Uncle Vanya" at the Almeida Theatre, earning high praise for her portrayal of Yelena. This production was also directed by Robert Icke.