She began her career as a journalist in the 1970s. Since then, she has held senior positions with The Irish Press and the Sunday Tribune, as well as serving as a political columnist at The Sunday Times and as the Political Editor of The Sunday Business Post. In 1998, she became the editor of Magill magazine. She resigned in September 1999 when the magazine's sister publication, In Dublin, was banned by the Censorship of Publications Appeal Board for advertising brothels and prostitution services. [dead link] O'Reilly was also a broadcaster on Raidió Teilifís Éireann (RTÉ) and Today FM.
In the course of her journalistic career, she won two awards: Journalist of the Year and Woman Journalist of the Year.
In a speech delivered in Dublin on 20 June 2006 to the Institute of Public Administration, O'Reilly criticised some Irish public bodies for retreating from dealing personally with the public through the use of call centres and the Internet. She cited the Revenue Commissioners in this context, pointing out that a significant proportion of the clients of these bodies are not computer literate and therefore the level of personal contact is inadequate as a consequence.