O'Connell's career breakthrough came when he starred in the independent prison drama "Starred Up" (2013). His portrayal of a violent teenager incarcerated in the same prison as his father received widespread critical acclaim; "Entertainment Weekly" critic Chris Nashawaty wrote, "O'Connell bristles with terrifying hair-trigger unpredictability. Watching him, you feel like you're witnessing the arrival of a new movie star." Peter Travers of "Rolling Stone" agreed, describing his "mad-dog incarnate" portrayal as "a star-is-born performance." O'Connell next starred in another acclaimed independent film, "'71" (2014), portraying a soldier deployed to Belfast at the height of political violence in Northern Ireland. He was director Yann Demange's first and only choice for the part. Writing for "Empire", Nev Pierce opined, "In a superb ensemble, O'Connell is outstanding," adding, "We know he can do violence, but here he holds the screen with no swagger—just a simple desire to survive." He received consecutive nominations for the British Independent Film Award for Best Actor.