Brigadier General O'Halloran was commissioned into the Communications and Information Services Corps in 1979 and has 41 years' service with a wide range of appointments at the tactical, operational and strategic levels of the Defence Forces. Prior to his current appointment (2016), Brigadier General O'Halloran served as Director of the Communications and Information Services Corps and wrote the First Defence Forces Cyber Strategy and developed the initial Defence Forces Cyber Capability. Brigadier General O'Halloran also served as Director of Logistics where he had responsibility for material and support logistics in the Army, Air Corps and Naval Service. He also held Senior Officer Appointments in Education and Intelligence.
Brigadier General O'Halloran has extensive overseas service having served 10 Tours of Duty on overseas operations. This includes 6 tours with the United Nations, 3 tours with the European Union and 1 tour with NATO. Notable tours were as Deputy of the Joint Implementation Commission with NATO in Kosovo in 2010 where he project managed the handover of the policing function of the Macedonian Border from NATO to the Institutions of Kosovo, Military Assistant to the Deputy Commander of the Irish led EUFOR CHAD mission in 2007 and Chief of Staff of the Irish led European Union Training Mission to Somalia in 2011-2012.
His professional military and civilian education is extensive. He is a graduate of the Irish Military College, the United Kingdom Defence Academy (2003) where he completed the Advanced Command and Staff Course (2003) and the Strategic Leadership Programme (2018), The National University of Ireland (2001) and the King's College London (2003) where he holds an MA in Defence Studies. He is also a graduate of the European Security and Defence College, the Folke Bernadotte Academy in Sweden, Case Western University USA (Culture) and the Public Affairs Institute Ireland (Corporate Governance). He is also a graduate of the Common Purpose Meridian Leadership Programme (2016) and a graduate of Harvard University, USA in Negotiation (2016)