Stirrup started his military career at the RAF College Cranwell in Lincolnshire on 1 April 1968 and it was from Cranwell that he received his commission on 31 July 1970. He was promoted to flying officer on 31 July 1971 with seniority backdated to 31 January, and to flight lieutenant from 31 July 1973. From 1973 to 1975, Stirrup was on loan service with the Sultan of Oman's Air Force. While in Oman Stirrup flew BAC Strikemasters during the Dhofar War in the close air support and interdiction roles, giving him valuable battle experience of the use of air power in counter-insurgency operations. After he returned to the United Kingdom in 1975, Stirrup was posted to No. 41 Squadron where he flew the SEPECAT Jaguar in the fighter reconnaissance role. Stirrup went on to serve in an exchange tour in the United States where he flew the all-weather tactical reconnaissance RF-4C Phantom.