Hayes was still a child when his father died, and he lived with his mother and grandmother and was educated at home by a governess, before going to a preparatory school in Dalkey and then in 1927 to St Columba's College at Rathfarnham, where his early interest in science began to develop as a hobby.
In 1947, Hayes returned to a Lectureship at Trinity College Dublin, where he continued his studies with Salmonella, developing his enthusiasm for bacterial genetics, and being awarded the DSc degree.
In 1950 he then moved to a senior Lectureship in bacteriology at the University of London Postgraduate Medical School at Hammersmith and began work on bacterial mating.
The importance of this discovery was quickly emphasised and widely recognised when he found that only a part of the genetic material was transferred from the donor strain (male) to the recipient.
In 1973, with the new Department at Edinburgh firmly established, Hayes accepted the Chair of Genetics at the Australian National University, Canberra, and went back to experimental work on Escherichia coli.