Oonagh is the Secretary General of the Department of Environment, Climate and Communications. Prior to taking up that role Oonagh served as Interim Chair of An Bord Pleanála, on secondment from her role as Deputy Secretary General in the Department of Justice with responsibility for civil justice matters, including immigration, courts and civil and family law.

Oonagh had previously held the position of Director General in the Workplace Relations Commission. Before that she worked at a senior level in a number of Government Departments, including the Departments of Public Expenditure and Reform, Environment and Foreign Affairs. In her previous time in the Department of the Environment she worked for a number of years on planning policy and legislation and on wildlife conservation.

Oonagh is a graduate of UCC, the College of Europe, Belgium and Poland, and the University of London. She was called to the Bar in 1996. She has an MSc in Business from the Smurfit School in UCD and has been an adjunct professor in the School of Law of UCC since 2017.