Irwin was born in Northern Ireland and moved to Southern Rhodesia (now Zimbabwe) in 1949 where he met Reay Smithers, then director of the National Museum of Southern Rhodesia in Bulawayo, and soon became collector with the museum.
[3] Irwin worked closely with Constantine Walter Benson, an authority on central African birds.
Together, they expanded the bird collection of the Natural History Museum of Zimbabwe until it became the largest both in Africa and in the southern hemisphere.
He contributed also to The Birds of Africa series (with Emil K. Urban, Stuart Keith and C. Hilary Fry) and has published more than 300 scientific papers, many of them in the museum's journal Arnoldia.
[9] Irwin was editor of Honeyguide, an ornithological journal of Bird Life Zimbabwe, for 24 years.