Fenian /ˈfiːnɪən/ was an umbrella term for the Fenian Brotherhood and Irish Republican Brotherhood (IRB), fraternal organisations dedicated to the establishment of an independent Irish Republic in the 19th and early 20th centuries.
The term Fenian is still used today, in Northern Ireland and less so in Scotland, where its original meaning has widened to include all supporters of anything Irish. It has also been used as a demeaning term for Irish people of the Catholic faith.
Fenianism (Irish: Fíníneachas), according to O'Mahony, is symbolised by two principles: firstly, that Ireland has a natural right to independence, and secondly, that this right could be won only by an armed revolution.
The term Fenianism was sometimes used by the [English] political establishment in the 1860s for any form of mobilisation among the Irish or those who expressed any Irish nationalist sentiments. They warned people about this threat to turn decent civilised society on its head such as that posed by trade unionism to the existing social order in the United Kingdom.
James Stephens, one of the "Men of 1848," (a participant in the 1848 revolt) had established himself in Paris, and was in correspondence with John O'Mahony in the United States and other advanced nationalists at home and abroad. This would include the Phoenix National and Literary Society, with Jeremiah O'Donovan (afterwards known as O'Donovan Rossa) among its more prominent members, which had been formed recently at Skibbereen.