Fenians, members of a secret Irish revolutionary society formed to win independence from Great Britain. The group was known variously as the Fenian Society, Fenian Brotherhood, and Irish Republican Brotherhood. The society was founded in New York City about 1858 by John O'Mahony, a revolutionist who had fled to the United States. Membership rose to about 250,000. The movement spread to Ireland and England, but the British government took steps to crush it.

In 1865, after the close of the American Civil War, the Fenians organized the “Irish Republic” in New York. They struck at Great Britain by making raids into Canada. In 1866 the Fenians made an unsuccessful attempt to seize Campobello Island, New Brunswick. The society broke up after O'Mahony's death in 1877.

The Fenians took their name from the Fianna, a group of Irish soldiers of the second and third centuries A.D. The many legends told about this group constitute the Fenian cycle of Irish literature.