born 1816, near Mitchelstown, County Cork, Ire.
died Feb. 6, 1877, New York, N.Y., U.S.
Irish-American founder of the U.S. branch of the Fenian Brotherhood (see Fenian movement ), an Irish secret revolutionary society.
An Irish resistance leader, he fled to New York in 1853, where he founded and led (1858–66) the Fenians. By 1865 the American Fenian group had become large and prosperous and was sending money and arms to Ireland. O'Mahony reluctantly backed the Irish Fenians' decision to stage military raids in Canada against the British, part of a scheme to hold Canada hostage for the cause of Irish freedom. The raids failed and he resigned, but he was later called out of retirement (1872–77). See also John O'Neill