Professor Irwin's key contributions have been the successful application of his
unrivalled knowledge of plant pathology to the development of commercially valuable, disease-resistant plant varieties for use by Australian fanners, and his distinguished leadership in the creation of a strategic research group rated by independent reviewers as the leading national and a significant international centre researching plant diseases and their control.

His high standing with knowledgeable farmers has been reflected in his selection by the rural industry research funds to advise them on national priorities for plant disease research. He is an authority on the fungal genus Phytophthora, one of the world's most economically important plant pathogens, and has achieved the rare honour for an Australian of election to Fellowship of the American Phytopathology Society.