Cullen was posted to the Irish embassy in
Paris as a
diplomat, and he developed an interest in the former trading links between Ireland and
France. In 1963, he joined the history department at Trinity College as a lecturer. He was made a Fellow in 1968. In 1972, he was appointed professor of Irish History. In 1970 he was a co-founder of the Economic and Social History Society of Ireland. He was later a Senior Fellow. A speaker of
Japanese, he has also written a history of Japan between 1582 and 1941. His book "An Economic History of Ireland Since 1660" has been reprinted and used widely as a textbook for students of Irish history. Cullen's close study of economic trends has led to a more nuanced understanding of aspects of Irish life in 1700–1850. A traditional view had been that the economic decline of Dublin from 1801 was linked to the 1800
Act of Union with Britain, and the loss of the
Parliament of Ireland and local political control. Cullen considered that the decline was inevitable, given the new steam-powered
Industrial Revolution, and would have happened even if no union had occurred, and if
Grattan's Parliament had managed to secure a high level of Irish autonomy. ==Selected works==