O'Neill was appointed assistant keeper of printed books at the National Library of Ireland (NLI) in 1947, leading to him developing an unparalleled knowledge of the manuscript material in the collections. Under the pen-name Lionel Thomas, he wrote for the Anti-Partition Campaign. As a fellow of the Library Association of Ireland, O'Neill published a pamphlet "Sources of Irish local history" in 1958. It was a collection of eight articles from the LAI's journal, "An Leabharlann", looking at types of sources and how librarians could use them to aid local historians written as "a user-friendly manual". The pamphlet remained a critical research tool for local historians for a long time. While at the National Library he acted as historical adviser on the documentary films about the 1916 Rising and the War of Independence, "Mise Éire" (released in 1959) and "Saoirse"(released 1961).