Keoghan's early life was marked by adversity. Raised in Summerhill, Dublin, he faced the challenges of his mother's struggle with drug addiction, a battle that persisted until her untimely death when Keoghan was just 12 years old. Alongside his brother Eric, Barry spent seven years in foster care, moving through 13 different homes before finding stability under the care of their grandmother, aunt, and older sister Gemma.
Despite a tumultuous upbringing, Keoghan's early exposure to the world of acting began in school plays at O'Connell School on Dublin's North Richmond Street. However, his mischievous nature resulted in a ban from participating. Undeterred, Keoghan embarked on a cinematic education, sneaking into films at Cineworld, Parnell Street, until he was eventually barred from the theater.
Determined to pursue his passion, Keoghan enrolled in The Factory (now Bow Street Academy), a local Dublin drama school. The journey, however, was not without its challenges, as he recalls not having enough money to afford the bus fare to The Factory at times.