He was born in London, and his parents were natives of Clare and Cork. After attending Crescent College Comprehensive in Limerick he graduated in medicine from University College Cork in 1982. He completed postgraduate medical training at the Johns Hopkins Hospital in Baltimore, the National Cancer Institute in Bethesda, and UCLA in internal medicine, immunology and gastroenterology. After four years on the faculty at UCLA, where he specialized in managing inflammatory bowel disease, he joined the pharmaceutical industry at Merck Research Laboratories. There he led the development of the groundbreaking drug Emend for the prevention of nausea and vomiting associated with cancer chemotherapy. A decade after its approval in 2003, no similar drug has been successfully developed.