He maintains a good relationship with the paternal side of his family, especially his grandfather Percy Thomas, who would visit McCormack and take him to the theatre.

[6][7][8][9] Upon graduating in 2014, McCormack was cast in his first professional role in the play The Grapes of Wrath at the Project Arts Centre.

[citation needed] The following year, he made his television debut in the RTÉ One soap opera Fair City as Pierce Devlin, a character he would play until 2016.

[10][2][11] In 2019, he starred alongside Olivia Cooke and Ben Hardy in Barnaby Thompson's comedy-thriller film Pixie and joined the cast of Peaky Blinders for its fifth series as Isaiah Jesus.

[17][18] McCormack appeared in Lee Isaac Chung's Twisters sequel and has forthcoming roles in the family thriller Anniversary[19][20] and Rian Johnson's Wake Up Dead Man.