Donough O’Brien was born on 24 June 1911 in Dublin, Ireland. He was brought up on the family’s country estate in County Limerick and later won scholarships to Bromsgrove School in England and to Trinity College Dublin, where he began a medical course in 1929. He graduated top of his year, with first class honours, in 1934. In 1939, he gained Fellowship of the Royal College of Surgeons (Ireland) and was awarded the Surgical Travelling Prize.

At the outbreak of war in the same year, he joined the British Army and spent 7 years as a surgeon in Scotland, Iraq, Egypt, Malta and Sicily. Early in the war, as a Duty Officer at Drymen in Scotland, he admitted and treated a “Captain Horn” for an injured ankle. It quickly transpired that the “captain”, who had flown a plane from Germany and landed in Glasgow, was in fact Rudolf Hess, who had come to the United Kingdom to try to negotiate a peace settlement with Winston Churchill.