[1] A condensed version of his Masters dissertation was published as a chapter in The great famine (1956) edited by R. D. Edwards and T. D. Williams.
Edwards relied on his post graduate student for much of the structuring of that work, and O'Neill was able to contribute a chapter of his choosing.
Sections relating to relief works were omitted, but O'Neill published these as journal articles, one of the few professional historians to study this aspect of the Irish famine at the time.
As well as contributing this chapter, O'Neill was also involved in the overall formatting of the volume, and devised the questionnaire sent out by the Irish Folklore Commission.
It was from this questionnaire that Roger McHugh wrote up a chapter on the folk memories of the famine, and statistical maps were devised from the data which also appeared in the volume.
[1] 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.
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".
O'Neill could both speak and write in Irish, but was heavily assisted by Donncha Ó Céileachair in getting this book to a publishing standard.
This biography drew on previously unused newspaper and archival sources, and is still regarded as the standard text on Lalor.
[1] Towards the end of his life, he expressed a wish to revisit and expand on his work on the famine, wanting to counter any revisionist narratives that may diminish the negative role of the English government in the events.
His last public appearance was to launch Gréithe Den Ghorta, a collection of Radió na Gaeltachta talks by Cathal Poirtéir.