Known as the Irish Republican Brotherhood when founded in 1858, the organization was keen on secrecy and American involvement. John O'Mahony, a Gaelic scholar, developed the name Fenians from the stories of the legendary warriors called Fianna. It soon became the common term for republicanism on both sides of the ocean.
Fenianism was organized into circles commanded by a center. Circles were further subdivided into cells under the command of captains who had sergeants and privates under their command. Lower-rank members knew only cell members, and everyone was sworn to secrecy. Despite all this, spies were rampant in the group and would later prove to be a problem.
The organization boasted about 100,000 members between its American and Irish chapters, but many more were financial donors. American donations in 1865 totaled about $228,000, and 1866 saw $500,000 go into Fenian coffers courtesy of Americans.
All during the American Civil War, Fenians held conventions to plan fund-raising, military tactics and the post-revolt government. Union soldiers, as a result, were heavily recruited after the war because the movement needed experienced troops. No officers actually joined, but many infantrymen did.
British intelligence was well into the republican groups by this time and began arresting American citizens in Ireland who were involved in Fenianism. In fact, when a sizable group of Civil War veterans set sail for the Emerald Isle in a boat named Erin's Hope, British intelligence knew the plan so well they arrested all the soldiers before anyone could even get off the boat.
Nonetheless, Fenian leaders still planned March 5, 1867 as the day of their great revolution. The British, knowing the plan beforehand, preventatively arrested 200 people in Dublin, but small skirmishes that resulted in violent riots still broke out in Cork and Limerick .
A special government commission convened to look into the happenings and charged 170 men with high treason. Many were sentenced to death, but many more were saved by appeals from the Roman Catholic Church and the American Government. Among the most publicized deaths were the hangings of three young Irishmen in Manchester following a trial heavy in anti-Irish sentiment. After being killed for the alleged murder of a police officer, the three became known as the Manchester Martyrs, and prayers went out from Catholic altars all over Ireland.
Even though the revolution was a failure, it continued the Irish belief that Britain would cede to force, among other lasting effects. On a grander scale, the situation also began the outline of the modern Irish problem, including: