As a medical undergraduate of Trinity College Dublin, Professor O'Neill spent a year in Marseilles as a volunteer with a NGO working with older people. He subsequently trained as a geriatrician in St. James's Hospital and the University of Bristol.
Following an appointment as consultant geriatrician in Selly Oak Hospital, Birmingham he returned to Dublin and currently is the senior academic in Medical Gerontology at the Trinity College Dublin campus at Tallaght Hospital.
All-Ireland Inspirational Life Award 2010
Irish Heart Foundation National Stroke Champion 2011
Joseph T Freeman Award, Gerontological Society of America 2014
Inaugural Presidential Medal and Lecture, Royal Academy of Medicine in Ireland 2015
FRCPI, AGSF, FRCP(Glasg), FRCP, FGSA, FRCPEdin