Irwin's experience is in the area of highway engineering, with particular emphasis on highway materials and pavement design. At Cornell he is the director of the Local Roads Program, which provides local highway agencies in New York State with technical assistance and training through Cooperative Extension. Irwin served for three and one-half years on the research staff of the Texas Transportation Institute at Texas A&M University prior to coming to Cornell in 1973. Before that, he taught civil engineering at California State University at Chico for three years.

A registered professional engineer, he has also worked for a large building contractor and a consulting engineering firm. He has been a consultant to industrial firms and government agencies, including the U.S. Army Corps of Engineers and the U.S. Forest Service. He recently spent a year working with the Strategic Highway Research Program of the National Research Council. He is a member of the American Society of Civil Engineers, the Transportation Research Board of the National Academy of Sciences, the Association of Asphalt Pavement Technologists, and the American Public Works Association.