Born in New York to Irish parents, O'Sullivan's family moved back to Ireland when he was a child, and he attended school in Ballinlough, Cork. From an Irish-speaking family, his first television roles were in Irish language productions for TG4. After roles in several BBC productions, and playing Bobby Storey in the 2017 film Maze, O'Sullivan moved to the US, obtaining recurring roles in NBC and Netflix series. Misty Button, in which he had a lead role, won the "best narrative feature" at the 2019 San Luis Obispo International Film Festival.
In 2021, he filmed a Sky Studios series titled Then You Run, which was released in July 2023.
As of 2022, O'Sullivan was reportedly working on a film titled The Lost Princess.

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