In The Gangs of New York, Herbert Asbury frequently mentions the Plug Uglies as one of the city’s notorious gangs, but offers very little detail about them: their leaders, when they flourished, the neighborhood that spawned them, their demise, etc. (See Gangs…, Chapter VI, Section 2 for one specific claim). The Plug Uglies of the 1850s were indeed a terrible gang of rowdies who engaged in street fights, beat and killed innocent civilians (“furriners”), and controlled polling places during city elections. Like the Bowery Boys, they were xenophobic thugs heavily aligned with the American (Know-Nothing) Party that flourished in that decade. However, the city that the Plug Uglies swaggered through and terrorized was not New York–it was Baltimore. They made one or two occasional forays to Washington, DC and Philadelphia, but not to New York City.
Asbury does not mention it, but there was one contemporary report that Plug Uglies had journeyed northward to join the 1863 Draft Riots in New York. It appeared in the New York Times on July 16, 1863, and referred to events of the previous day. The Times building itself was under siege; what little information the editors received was from police officials. Without trained reporters to witness events, they relied on hearsay.
“The scoundrels and roughs–the Blood Tubs and Plug Uglies of Baltimore, and the Schuylkill Rangers and other rowdies of Philadelphia are reported to have come to the city in large numbers to make common cause with the Dead Rabbits, Mackerelvillers, and other leading spirits of the riot in their work of carnage and plunder.”
Note the phrase “are reported,” which means, “we were told this, but can not confirm it.”
The Plug Uglies did not exist at that point. They had been broken up by a reform local government crackdown in Baltimore in 1860, and what remnants survived fled the arrival of Union troops when the Civil War started and headed south to Virginia to support the Confederacy. The Dead Rabbits–largely Irish and Catholic–would have been the traditional mortal enemies of Plug Uglies.
It seems that whenever street protests devolve into riots, there is always a claim made by authorities that violent outside agitators have arrived as reinforcements.