Born in County Carlow, O’Neill is educated at St. Mary’s Knockbeg College and University College Dublin (UCD). While assistant keeper of the National Library of Ireland, he is asked to undertake the work on de Valera. Frank Gallagher, head of the Government Information Services and later a member of the library’s staff had been working on a biography for several years but dies in 1962 without completing the work.
De Valera knows of O’Neill’s reputation as a historian and asks him to undertake the project. A contract is signed with the publishers in 1963, and O’Neill moves to Áras an Uachtaráin to work on the book. He is later joined by Lord Longford as co-author.
After the completion of the de Valera work, O’Neill is appointed lecturer and later professor of history in University College, Galway. On his retirement, he returns to live in Dublin, where he renews his association with the National Library, becoming a strong supporter of its expansion.
O’Neill continues historical research until shortly before his death. He discovers evidence that the 1916 Proclamation of the Irish Republic was signed by the seven signatories at the home of the president of Cumann na mBan, Jennie Wyse Power, in Henry Street, Dublin, before the Easter Rising and not merely printed in Liberty Hall from an unsigned manuscript on Easter Sunday.
O’Neill is survived by his wife, Marie, and six children. His funeral Mass takes place at St. Joseph’s Church on March 5, followed by his interment at Shanganagh Cemetery.
(From: “Biographer of de Valera dies at 74,” The Irish Times, Monday, March 4, 1996)