The Fenian Brotherhood, the Irish Republican Brotherhood’s US branch, was founded by John O’Mahony and Michael Doheny, both of whom had been “out” (participating in the Young Irelander’s rising) in 1848. Members were commonly known as “Fenians”. O’Mahony, who was a Celtic scholar, named his organisation after the Fianna, the legendary band of Irish warriors led by Fionn mac Cumhaill.

The Fenian Brotherhood trace their origins back to 1798 and the United Irishmen, who had been an open political organisation only to be suppressed and became a secret revolutionary organisation, rose in rebellion, seeking an end to British rule in Ireland and the establishment of an Irish Republic. The rebellion was suppressed, but the principles of the United Irishmen were to have a powerful influence on the course of Irish history.

In the face of nativist suspicion, it quickly established an independent existence, although it still worked to gain Irish American support for armed rebellion in Ireland. In 1863, Stephens founded a newspaper called the ‘Irish People’. He wanted to make as many people as possible aware of what the Fenians stood for. O’Mahony did not approve of this move as he felt such a paper would attract even more attention to the movement from the British government based in Dublin. He preferred the movement to develop in secrecy.

Initially, O’Mahony ran operations in the US, sending funds to Stephens and the IRB in Ireland, disagreement over O’Mahony’s leadership led to the formation of two Fenian Brotherhoods in 1865. The US chapter of the movement was also sometimes referred to as the IRB. However, as the Fenian movement grew, so did the difficulties of keeping it organised. This had proved difficult because of the Irish-American geographic split and the problems of communications. But the two founders – O’Mahony and Stephens – disagreed on how the movement should develop.

Another problem faced by the Fenians was that the Roman Catholic Church was generally not supportive of them. The power of the local priests was great and their influence within a local community, and especially among the older members of such communities, meant that they could undermine whatever influence the Fenians tried to establish.

After the 1867 rising, IRB headquarters in Manchester opted to support neither of the duelling American factions, promoting instead a new organisation in America, Clan na Gael. The Fenian Brotherhood itself, however, continued to exist until voting to disband in 1880.

In 1881, the submarine Fenian Ram, designed by John Philip Holland for use against the British, was launched by the Delamater Iron Company in New York.