Fenianism – name popularly given to militant Irish republicanism, in opposition to constitutionalism and its limited goal of Irish Home Rule.
"Fenians" were the bogeymen of British politics from the 1860s to the 1910s (when their place in public disdain was usurped by "Sinn Feiners"). The name retains its power to this day: "Fenian" is a usual term of abuse for any nationalist (or indeed Catholic) in Northern Ireland.
The movement derived from two sister organizations, one based in America, the other in Ireland. The Irish Republican Brotherhood was founded in Dublin on St Patrick's Day 1858 with the ultimate aim of achieving an Irish Republic by physical force. Funds were provided by Irishmen in America who, at the same time, founded the Fenian Brotherhood. Although the American and Irish organizations remained separate, the entire republican movement took the name "Fenianism", and advocates were popularly known as Fenians.
In Ireland, the Irish Republican Brotherhood was a secret oath-bound society, organized into "circles" (analogous to regiments) headed by "centres" (analogous to colonels). Their objective was based on the two principles: that Ireland had a natural right to independence, and that this right could be won only by armed revolution. Members recognised the Supreme Council of the IRB as the "Provisional Government of Ireland". In America "Irish Republic" bonds were issued to raise funds. The watchword was "Soon or Never".
Denounced by the Irish church and by the Vatican, ridiculed for its early ineptitude (its "rebellion" of 1867 was a miserable washout, as were the later Fenian "raids" into Canada), and reviled for its "dynamiting" campaign in England, it nevertheless maintained a grip on the Irish imagination. Like its constitutionalist counterpart, the Irish Parliamentary Party, it infiltrated all levels of society – cultural, sporting, economic.
By 1915 its members had infiltrated the leadership of the Irish Volunteers. It was these "secret" leaders (including Pearse and Connolly) who precipitated the Easter Rising.
Fianna — a legendary group of heroic warriors who "were charged to defend the sovereignty of Ireland against external enemies, both natural and supernatural" – Oxford Dictionary of Celtic Mythology.
"Fenian" is an anglicized form.