"[2] A chance discovery in 1929 of two inscription stones in the praetentura[6] at Birdoswald led Birley to suggest redating the Wall periods.

This discovery, and Birley's redating, impacted the study of Roman Britain tremendously inasmuch as it "has formed the basis of all subsequent work on the chronology of Hadrian's Wall.

"[3] In 1929, having developed a keen interest in the Wall through his excavations, Birley succeeded in buying the Clayton Estate[3] at Chesterholm[2][5] that had belonged to Anthony Hedley, the 19th century antiquary.

In 1931, at the age of 24, he became a lecturer[2][5] at the University of Durham, "already with an impressive record of excavation on Hadrian's Wall, partly under the guidance of F.G. Simpson,"[5] whom he would replace upon the latter's stepping down as Director of Field Studies.

[5] Through trips to Germany and Switzerland, he began to expand his expertise in samian pottery, epigraphy, and his primary field of interest, the Roman army.

[5] Also, during his years at Durham, he would influence such notable students as George Jobey and John Gillam, each of whom would go on to hold prestigious university teaching positions.

[7] His expertise and work in this field served to make Birley the founder of the "Durham School" of archaeology having attractedseveral highly-talented and influential postgraduate students to the Department, whose research has … "epitomised, and to a large degree set the agenda for, British work on the Roman military for much of the twentieth century.

[9] In the New Years Honours of 1943 Birley, by then promoted to the rank of captain and temporary major, became a member of the military division of the Order of the British Empire (MBE).

[13] Major Birley reached the age limit (50 years) for service on 12 January 1956 and retired from the army on that date, being granted the honorary rank of lieutenant-colonel.