[3] He owned O'Neill Motors,[4] a Dublin garage acquired in 1959 by Ryan's Car Hire.
[6] During World War I he chaired the All-Ireland Munitions and Government Supplies Committee, which tried with limited success to secure war-related contracts for Irish firms.
[2][7][8] He served on Lord Balfour of Burleigh's 1916–1917 Committee on Commercial and Industrial Policy, dissenting from its final report in particular due to Ireland-specific issues.
[4][16][17] O'Neill was elected to the Seanad at a by-election on 30 March 1925 to fill the vacancy created by the death of George Sigerson,[18] finishing one vote ahead of Patrick McCartan.
[3] He stood unsuccessfully in the 1927 by-election to fill the vacancy created by the death of Stephen O'Mara, finishing fifth behind P. J.