Gaelic League (1893–1943): foundation documents; notes on structure and organisation; articles and correspondence concerning policy and development; circulars and resolutions of the Irish Language Congress (1894); accounts of conventions and feiseanna and the establishment of local branches and county and district committees; lists of members; correspondence with the Society for the Preservation of the Irish Language; correspondence with contributors to the Gaelic Journal and An Claidheamh Soluis; lectures, articles, notes and correspondence on Irish grammar, vocabulary, pronunciation and dialect; articles on the teaching of Irish, the decline and revival of the language, the quality of Irish literature, and the development of a modern vocabulary.

Irish Volunteers (1913–54): draft policy documents and manifestos outlining organisation and objectives; copy circulars from the provisional committee concerning the split with the National Volunteers; correspondence with Bulmer Hobson on organisational matters, rule-drafting, fund-raising, and the purchase of equipment; reports on central and local organisation, recruitment, levels of membership, finance, and the local effects of the split with the National Volunteers; correspondence concerning the serving of expulsion orders upon members under the Defence of the Realm Regulations; correspondence concerning the dismissal of public servants for involvement in the Volunteers; retrospective accounts of foundation, development and activities.

1916 Rising: copies of statements by participants; memoir by MacNeill on the Rising and his arrest; correspondence and notes relating to his court martial; personal correspondence while in prison; police intelligence documents relating to surveillance on MacNeill, searches of his house, his arrest and detention; prison correspondence of Turlough MacNeill; correspondence and memoranda concerning the Irish Convention (1917–18); correspondence concerning the anti-conscription campaign and the Mansion House Conference (1918).

Minister for Finance and Minister for Industries in the First Dáil, Ceann Comhairle in the Second Dáil, Minister without Portfolio and Minister for Education in the Provisional Government, Minister for Education in the Executive Council (1919–25): First Dáil reports on industries, labour and local government; memoranda concerning Dáil finance; correspondence as Ceann Comhairle with T.D.s; articles and speeches, notes and correspondence concerning the Anglo-Irish Treaty; correspondence with the Treaty Election Committee; correspondence with the Constitution Drafting Committee, printed and typescript drafts of the Constitution (1922) and memoranda and notes on drafts; Minister for Education’s copies of reports, memoranda and circulars from the Offices of the Chairman and General Secretariat of the Provisional Government, President and Secretary of the Executive Council, and Commander-in-Chief, Free State Army; copies of captured republican documents; Department of Education memoranda for the Minister; copies of memoranda from other ministers and departments and related correspondence; Army Enquiry Committee correspondence and reports; correspondence and memoranda concerning the Imperial Conference, London (1925).

Irish Race Congress, Paris (1922): programme; notes and reports from the Cabinet nominees in the Irish delegation; correspondence concerning the activities of the anti-Treaty delegates; Congress minutes and resolutions; minutes of the Council of the Irish Race Organisation (Fine Gaedheal) established by the Congress; plan of organisation; report of an Irish government investigation into the affairs of Fine Gaedheal.

North-East Ulster and the Boundary Commission (1922–5): articles and speeches by MacNeill on the Ulster question; memoranda on delays in the implementation of Article XII of the Treaty, on the refusal of the Northern Ireland government to appoint a representative to the Boundary Commission, and on procedure at the Commission; memoranda and legal opinions on the power of the Commission to transfer Free State territory to Northern Ireland; reports from the North-East Boundary Bureau on the political situation in Northern Ireland; memoranda on the policy to be adopted by the Executive Council in relation to the Commission and on points for a settlement of the boundary question; report of a commission appointed by the Executive Council to consider an offer to North-East Ulster; correspondence concerning arrangements for sittings of the Commission; transcripts of evidence; notes on hearings; diary of the Irish Boundary Tour (December 1924); draft statement of the Executive Council’s case; memoranda from Free State government departments for meetings of the executive Council on the administrative steps necessary to give effect to any decisions by the Commission on the transfer of territory; correspondence concerning MacNeill’s resignation from the Commission.

Irish Manuscripts Commission (1929–54): notices, agendas and minutes of meetings; memoranda on the establishment of the Commission; reports on record groups and manuscript collections; correspondence and reports on proposals for publication; reports on the progress of editorial and indexing projects; correspondence with the Department of Finance concerning specific publications and the scope and direction of the Commission’s work; correspondence with the Department of the President of the Executive Council concerning appointments to the Commission.

National University of Ireland: articles on the university question and on the establishment of the National University of Ireland with particular reference to the position of Irish studies within the university (1907–9); correspondence, reports and notes on salaries, building costs and general finance in University College Dublin (1908–39); proposals for the establishment of various institutes of Irish studies and folklore; correspondence and diaries concerning his American lecture tour (1930) and the promotion of Irish Studies in the United States; reports concerning Omeath Irish College (1914–32); correspondence and reports concerning the International Congress of Christian Archaeology, Rome (1938); correspondence with the Department of Finance concerning the establishment of the Institute for Advanced Studies (1940–2).

Essays, articles, notes and correspondence on topics relating to the Irish language, Celtic studies, Irish history, folklore and archaeology including: Irish manuscripts; early verse; Ossianic literature; Latin texts; inscriptions in Gaulish, Irish and Pictish; linguistics; philology; topography; early Christianity in Britain and Ireland; early Irish laws; Druids; ethnic groups in the British Isles; kingship and genealogy; annals and chronicles; Saint Patrick; the discipline of archaeology and individual finds in Ireland.

Autobiographical memoir covering his life from 1868 to 1927. Personal documents including testimonials and letters of application for positions (1882–1928); correspondence with members of his family (1890–1944); news cuttings of articles by and about MacNeill (1899–1978); correspondence concerning MacNeill’s career and his papers mainly in relation to research being carried out.