Answer: The Fenians. 

Founded in the late 1850s, the Fenian Brotherhood was the American offshoot of the Irish Revolutionary Brotherhood, a radical organization that believed in Irish independence and the establishment of it through armed conflict.

In the late 1840s, Philadelphia's Irish population numbered in the tens of thousands, many of whom had immigrated to the city after the Irish potato famine of 1846-1847. Large segments of this local population firmly believed in Irish nationalism and were willing to fight for it. Nationalists movements such as the "Young Ireland" movement of late 1840s and early 1850s, served as inspiration for their cause.

A branch of the Fenian Brotherhood in Philadelphia had been established by the 1860s, and the city hosted the Fenian's first national convention in 1863. A Philadelphia printer by the name of James Gibbon was elected then a director of the organization. Interesting plans were laid at that convention that involved taking as members Irishmen from the Union Army in preparation for future military actions against England.

The Fenian Brotherhood in the U. S. was eventually superseded by Clan Na Gael, which took up the fight for Irish independence in 1867.  The brotherhood continued some underground operations, but slowly dissolved over the course of the 1870s and 1880s.

HSP's library contains a wide assortment of publications on the history of the Fenians and the Irish in Philadelphia. Titled include Recollections of an Irish rebel: The Fenian Movement (DA 958 .D4 A3), Troublous times in Canada:  history of the Fenian raids of 1866 and 1870 (F 1032 .M115), and James Stephens…[and] the Origin and Progress of the Fenian Brotherhood (Gs .8). We also have manuscript collection from historian Dennis Clark (MSS37, MSS177) whose papers primarily concern Irish American history.