Regan was born on 24 February 1982 in Kilkenny, and grew up in Tipperary. She attended University College Cork (UCC), since the majority of her family live in Cork. At UCC, she studied English and History. In 2004, at the age of 22, Regan made her stage debut as Charlotte in a one-woman production of The Yellow Wallpaper. She described the show as the "catalyst" for her motivation to begin acting professionally, stating: "I wasn't sure if I had the ability or the stomach for acting and The Yellow Wallpaper helped me to figure that out". Afterwards, she told her parents that she wanted to study acting at the Royal Academy of Dramatic Art (RADA) in London, to which she stated that they were "not surprised". She then borrowed the money for the train fare to Dublin from her parents in order to audition for a place at RADA, to which she was accepted.
