Ronan made her screen debut on Irish national broadcaster RTÉ, in the 2003 prime time medical drama "The Clinic" and then appeared in the mini-serial "Proof". During the same time, Ronan auditioned to play Luna Lovegood in "Harry Potter and the Order of the Phoenix", a role she eventually lost to Evanna Lynch. Ronan's first film was Amy Heckerling's romantic comedy "I Could Never Be Your Woman", which was filmed in 2005. It was theatrically released in a few international markets in 2007, but given a direct-to-video release in the US in 2008, after it struggled to attract financing and several deals disintegrated during its post-production. In the film, Ronan portrayed the daughter of Michelle Pfeiffer's character and Paul Rudd co-starred as Pfeiffer's love interest. Joe Leydon of "Variety" labelled the film "desperately unfunny" but considered the interplay between Ronan and Pfeiffer's characters to be among the film's highlights.