Fenians
2 of the 7 encyclopedias on this shelf carry an entry for Fenians. Both are reproduced below, so you can see where they agree and where they differ.
Collier's New Encyclopedia (1921)
an Irish secret society named from an ancient military organization of Ireland that became extinct in the 3d century. The Fenian society was formed in the United States probably in March, 1858, by the refugees who crossed the Atlantic after the unsuccessful outbreak of 1848, and had for its object the expulsion of the British Government, or even the Saxons from Ireland, and the conversion of that island into an independent republic. Its originator divided it into district clubs called circles, each with a president called a center; the whole organization being ruled over by a senate, over which a "head center" presided. Its members had to take an oath before being intrusted with its secrets. In January, 1864, they began to attract notice in Ireland, and the next year some of them were seized and imprisoned. Between 1865 and 1867 they made various outbreaks. In 1866 they captured British vessel, and made a raid into Canada, but were defeated by the volunteers and censured by President Johnson. In 1867 they unsuccessfully attempted an attack on Chester Castle in England, made other risings, and on Dec. 13 blew in the wall of Clerkenwell prison, killing and wounding a number of innocent people living in adjacent houses. A second Fenian raid into Canada took place in 1870, but was repelled by the militia. The basis for all the Fenian operations was America, where, in 1865, 600 Fenian representatives held a congress. First and last, many Fenians were captured and imprisoned by the British Government, most of whom were after a time released. The organization seemed to become dormant about 1874, and various persons who had been connected with it joined the "Invincibles," formed some years later for the purpose of assassinating government officers or others obnoxious to its members or its chiefs, but not much was known of this latter organization until the murder of Lord Cavendish called attention to them.
Aiton's Encyclopedia (1910)
an organization formed in America and Great Britain in 1859 to secure the independence of Ireland. The Fenian society was organized on the model of the United States Constitution with local, district, and state circles, and a congress. Fenian congresses met in Chicago and Cincinnati in 1865. As many as 60,000 Irishmen are said to have been enrolled. In 1866 several Fenian attempts were made to invade Canada. Two companies actually crossed the Canadian frontier, but were driven back. The leaders were arrested by the United States authorities and sent home on parole. Several Fenian outbreaks occurred in Great Britain, and Fenian rioters were arrested. The repeal of obnoxious legislation and a more liberal treatment of Ireland have allayed Irish discontent. Dissensions within the organization have broken it up. As to the origin of the name, Fingal and his Fenians are the legendary heroes of Ireland, corresponding to King Arthur and his Knights of the Round Table. Their deeds are the theme of many a legendary tale and song. MacPherson's Ossian was the legendary son of this famous chieftain, Finn or Fingal, for whom, we may add, Fingal's Cave is named.