Irish-born military leader of the American branch of the Fenians (Fenian), an Irish nationalist secret society.

O'Neill immigrated to the United States at the age of 14 to join his mother and older siblings at their home in Elizabeth, N.J. He attended school for a year and then held a number of jobs. After serving as a Union cavalry officer during the American Civil War, O'Neill—disappointed in his failure to be promoted—resigned from the army. He settled in Tennessee, where he was working as a claims agent when he became interested in the Fenian scheme of capturing Canada and holding it hostage in order to secure Irish freedom from Great Britain.

O'Neill gathered troops from his area and in 1866 led a Nashville detachment to Buffalo in preparation for the assault on Canada. With 600 men he crossed the Niagara River and took the Canadian village of Fort Erie. Before British troops could retaliate, O'Neill's force defeated a group of Canadian militiamen at the Battle of Limestone Ridge and fled back to the United States. There he was arrested for violating the neutrality laws, but the charges were dropped.

By 1867 O'Neill was one of the top Fenian leaders. For years he threatened to attack Canada once again, and he finally did so on May 25, 1870. But when the Canadians repulsed his raid along the Vermont border, O'Neill's troops fled, and he was arrested by a U.S. marshall. Convicted and sentenced to two years in prison, O'Neill served only three months before being pardoned by President Ulysses S. Grant.

O'Neill promised that he would not again attack Canada, but then—without the support of the main Fenian organization—he launched a raid on Manitoba on October 5, 1871. After capturing a Hudson's Bay post, he was taken into custody by U.S. troops. He won a quick release from the courts. Finished now with the Fenians, O'Neill took a job attracting Irish immigrants to settle a tract of land in Nebraska.