Costello was born on 4 July 1904 in Cloughjordan, County Tipperary. Costello became involved in the Irish War of Independence of 1919-1921, after seeing his father, a school teacher, arrested by the Black and Tans.He served with the Old Ireland Republican Army as an Intelligence Officer with no.1 Tipperary brigade.
Costello later attended the American Staff college, and became a leading light in developing tank warfare strategy with the Imperial General Staff in London. He attended the United States Army"s Command and Staff College at Fort Leavenworth from 1926 to 1927.
His godfather was Thomas MacDonagh, who signed the Proclamation of the Irish Republic in 1916. Costello joined the Irish National Army in 1922 and fought in the Irish Civil War of 1922-1923. Michael Collins promoted him to Colonel-Commandant when Costello was still only eighteen years old.
He served as National Army Director of Intelligence from 1924 to 1926.
Costello advised the Free State on establishing its own Irish Military College. In a series of articles in the Irish military journal An t-Oglach, Costello predicted the advent of blitzkrieg warfare.
He was appointed Director of Training in 1931 and Commandant of Irish Military College in 1933. During The Emergency, he commanded the Irish Army"s First Division, which was primarily responsible for the defence of the south coast of Ireland, as O/C Southern Command from 1940.
The division, a volunteer force, had able personnel, but was poorly equipped.
In Costello"s words,
The platoons made silent advances during dark nights over difficult and unknown country."
In 1944, one of Costello"s units carried out a 44-mile (71-km) march carrying 40-pound (18-kg) packs in 11 hours. This feat was later deemed a "world record" for such a march in peacetime. Costello was instrumental in founding the publication for irish defence studies, known as An Cosantoir from 1941.
Costello was promoted to Major General in 1941 and to Lieutenant General in 1945.
He retired from the Army in 1946, after which he became the Managing Director of the Irish Sugar Company. He died on 20 October 1986.
"Given the inadequate armament and signal equipment of the Forces, training concentrated on attaining superiority over a potential invader in night operations with small forces expected to yield capture of enemy arms and ammunition, in cross country mobility and in marksmanship and the use of mines and explosives generally.
