Dr Fehily has worked as a Nutrition Scientist in the Medical Research Council (MRC), a Company Nutritionist in the food industry and is currently Consultant Nutritionist to Tinuviel Software. Research included ischaemic heart disease and its risk factors. As part of this work, this team conducted the first randomised controlled trial to show that eating oily fish reduces risk of death from a heart attack (Burr, Fehily, Gilbert, et al, 1989).

With Tinuviel Software, she is responsible for the nutritional content of all company nutritional software design. She was involved in the development of the company's QBuilder program. This enables individuals to design and build their own food intake questionnaires.