He was born in Manorcunningham, County Donegal, Ireland. In 1875, he studied medicine at Trinity College Dublin .
Irwin joined the Royal Army Medical Corps at Netley in 1881 and was commissioned as surgeon captain posted to Dublin in February 1882. He was then posted to India in September 1883.
He was subsequently posted to Dublin in 1890, Gibraltar in 1891 and Brighton in 1897.
In February 1894, he was promoted surgeon major .
Posted to Brighton in 1897. Posted to Sudan, Medical Officer, Atbara, Battle of Omdurman 1898. Posted to Expedition in Crete 1899. Served as Medical Officer on troopship HMS Verona and then posted to Dublin also in 1899.
Irwin was posted to South Africa in 1900, for service during the Second Boer War, and was promoted to lieutenant colonel on 4 February 1902. Following the end of hostilities in June 1902, he left Cape Town for England and returned to Southampton in early August.
He was then posted to Aldershot, and became Assistant to Surgeon General McNamara in 1903.
Posted to War Office as Assistant Director General 1906. Posted to Tientsin, North China 1910.
Promoted Colonel and posted to Hong Kong as Deputy Director of Medical Services, South China 1911.
Travelled around the world 1913.
London establishing the new King George V Hospital, Waterloo 1914. France Deputy Director of Medical Services, Rouen 1915. Director of Medical Services 3rd Army 1916.
Promoted Temporary Surgeon General April 1916.
He returned to England in 1919.
After World War I he retired to Bideford, North Devon where his wife had purchased a house. He died on 7 November 1938(1938-11-07) (aged 80).
He married Nora Conlan, daughter of Thomas Conlan, Q.C., of Allahabad, 26 July 1884 and they had a daughter born 31 October 1885, Edna Florine.