McCarthy's first major film role came in 2017, when he was cast as the lead in the film The Drummer and the Keeper. The film is about a teenage boy with Asperger's syndrome who strikes up an unlikely friendship with a reckless musician. McCarthy's performance was praised by critics, and he won the award for Best Actor at the Galway Film Festival.
In 2021, McCarthy starred in the film The Tragedy of Macbeth, directed by Joel Coen. The film is an adaptation of the Shakespeare play, and McCarthy played the role of Seyton, a loyal servant to Macbeth. McCarthy's performance was praised by critics, and he was nominated for the Golden Globe Award for Best Supporting Actor – Motion Picture.
McCarthy is currently filming the upcoming film The Banshees of Inisherin, directed by Martin McDonagh. The film is about two lifelong friends who become enemies after one of them makes a mistake. McCarthy will play the role of Padraic Súilleabháin, a young man who is caught up in the conflict between the two friends.