She was previously known as Marie Smyth but adopted the surname of her husband, Alan Johnston Breen, after his death on 7 July 2005.
On 13 February 2007, The Guardian published an article 'The abuse of research'[5] by Breen Smyth and Jeroen Gunning in which they argue that the sources of funding and the political agendas of think tanks should be taken into account when interpreting their findings, and may mean that their reports need to be treated with care.
[9] Breen-Smyth opposed the appointment of four commissioners instead of one, which required new legislation[10] delayed by an unsuccessful judicial review brought by victims' representative Michelle Williamson.
[11][12][13] Breen-Smyth separately launched a legal challenge to the appointments in the employment tribunals,[14][15] claiming discrimination on the grounds of religious belief, political opinion and sex, but this was rejected in February 2011.
In the 1980s, Breen-Smyth co-founded "Derry Well Woman", a free health and support service for women based in the North West of Ireland.