"Confederacy of Fenians" is a pleasing story of what might have been. "What if" Great Britain had entered the U.S. War of Independence in 1863 after Confederate forces defeated Union soldiers at Gettysburg, which also did not happen - though well might have and actually almost did.
Author James Nealon calls it a "yarn," which it is. A good read. Instructive, too. How many readers recall the crucial role that Irish immigrants fleeing famine at home played in the Union cause? No fiction, that. The justly famed Irish Brigade faced down Pickett's charge, effectively ending the Battle of Gettysburg.
The Fenians in the title refer to Irish organizations formed in the late 19th century to build support for achieving their own country's independence from England. Which of course eventually did happen. But that's another story.