Donal qualified from University College Dublin in 1989 before moving to Hammersmith Hospital in London, where he completed research with Professor Sir Stephen Bloom on how the brain controls appetite. He moved to his current position in 1999, establishing two hospital-based multidisciplinary treatment teams for the management of adult obesity and gender dysphoria.
Donal is a member of the Department of Health Policy Group on Obesity and chaired a group carrying out a health impact assessment on the potential benefits and harms of a tax on sugar-sweetened drinks. He has presented to EU Ministers for Health and the Director General of the World Health Organisation on the importance of the prevention of childhood obesity. Donal has specific research interests in obesity and gender and has published over 250 research papers. He served as National Clinical Lead for the Management of Obesity with the Health Service Executive until February 2026.