Born in Kinsale in 1882, O'Neill was the son of James O'Neill, a Kinsale merchant and member of the first Cork County Council. He was educated at the Christian Brothers School in Youghal, the Presentation Brothers School in Kinsale, Mungret College, and the Royal University of Ireland, where he was awarded a B.A. degree in 1901.
He inherited his father's business in Kinsale. He was one of the founders of the Irish Master Bakers' Association. He campaigned to have electricity brought to Kinsale, and succeeded in 1920. From 1925 to 1928 he served on Cork County Council. He and Rev. Patrick MacSwiney, curate in Kinsale from 1927 to 1940, supported each other in their endeavours to improve the economic, social and cultural situation in the depressed town.
O'Neill was an unsuccessful Cumann na nGaedheal candidate at the September 1927 general election in the 5-seat Cork West constituency. He stood again at the next opportunity, at the 1932 general election, and was elected as a Teachta Dála (TD) for Cork West. He was re-elected at the next three general elections, before losing his seat at the 1943 general election to the Clann na Talmhan candidate Patrick O'Driscoll. He served as Leas-Cheann Comhairle from May 1939 to May 1943.
He was re-elected a year later, at the 1944 general election, unseating his Fine Gael colleague Timothy O'Donovan. The constituency was reduced to a 3-seater at the 1948 general election, and O'Neill was not re-elected. He did not contest any further elections.
- "Eamonn O'Neill". Oireachtas Members Database. Retrieved 9 January 2011.
- Tim Cadogan and Jeremiah Falvey, A Biographical Dictionary of Cork, Dublin 2006, p. 266
- "Eamonn O'Neill". ElectionsIreland.org. Retrieved 9 January 2011.
- See Ruth Fleischmann, Ed., Joan Denise Moriarty: Ireland's First Lady of Dance, Cork 2012, p. 20.