John O'Neill was born on March 8, 1834, at Drumgallon in the parish of Clontibret, County Monaghan, Ireland. His father died before the boy's birth. John remained in his native parish until 1848, when he emigrated to America to join his mother and her elder children, who had settled in Elizabeth, New Jersey, some years before.
John O'Neill obtained the elements of an education in Drumgallon, County Monaghan, Ireland, until 1848, later he emigrated to New Jersey, United States, to join his mother, where he attended school for another year.
After schooling John worked successively as a shop clerk, a traveling bookagent, and proprietor of a Catholic bookshop in Richmond. In 1857 he joined the 2nd United States Dragoons for the "Mormon War. " In Utah, it appears, he deserted, and made his way to California, where he joined the 16t Cavalry, with which he was serving as a sergeant when the Civil War broke out. With this regiment he returned to the East to join the Union army and fought in the Peninsular campaign. In December 1862 he was appointed second lieutenant in the 5th Indiana Cavalry, and was promoted first lieutenant in the following April. He soon acquired the reputation of being an unusually active and daring officer. He distinguished himself near Glasgow, Kentucky, in June 1863, and again shortly afterward at Buffington Bar, in the course of Morgan's Ohio raid. On December 2 he was severely wounded at Walker's Ford. Feeling that he was being passed over for promotion, in the spring of 1864 he resigned from his regiment and was appointed captain in the 17th United States Colored Infantry, only to leave the service in November.
While working successfully as a claims agent in Tennessee, John O'Neill became interested in the plans for an invasion of Canada proposed by the party headed by W. R. Roberts in the Fenian Brotherhood, which eschewed politics in favor of militant action to expel the British presence in Ireland. He acted as a Fenian organizer in his district and in May 1866 led a detachment from Nashville to take part in the attack. Finding himself in command of the raiding party at Buffalo he led a force of 600 men, by his account, across the Niagara and occupied the Canadian village of Fort Erie. The next day he defeated a small column of Canadian volunteers near Ridgeway, and that night escaped from Canada with his men by boat before British troops closed in on his position. The raiders were arrested by a United States gunboat but released a few days later, and a charge of breach of the neutrality laws brought against O'Neill was dropped.
A few months later John was appointed "inspector-general of the Irish Republican Army, " and at the end of 1867 he replaced Roberts as president of his branch of the Brotherhood and proceeded to prepare for another attack on Canada. There were obstructionists within his own organization, but his threats caused much alarm in Canada. In 1870 he quarreled with his "senate, " and only a fraction of the Fenian organization supported him when on May 25 he attempted a raid at Eccles Hill on the Vermont border. His men fled when the Canadians opened fire, and he himself was arrested by a United States marshal and sentenced to two years' imprisonment, but he was released by presidential pardon after three months.
O'Neill declared he would not again trouble Canada but was persuaded by W. B. O'Donoghue, formerly a member of Louis Riel's rebel government at Fort Garry, to attack Manitoba. The Fenian council, now mistrusting O'Neill, rejected the scheme, but he made the attempt with a few adherents on October 5, 1871. He seized the Hudson's Bay post at Pembina (on territory then disputed between Canada and the United States) but was immediately arrested by United States troops. He was released by the American courts.
Later he became agent for a firm of land speculators who desired Irish settlers for a tract in Holt County, Nebraska. While thus engaged he died at Omaha.
John O'Neill was leader of the Fenian Brotherhood.
John O'Neill rejected assassination as an Irish weapon, insisting on "fair and honorable fight"; and though Fenianism was condemned by the church, he claimed to be a devout Catholic. His egotism made it hard for him to work with others.
In 1864, John O'Neill married Mary Crow.
