 Complete the following phrase:  ONeill 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, ONeill published a pamphlet Sources of Irish local history in 1958. It was a collection of eight articles from the LAIs 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 