They served on opposite sides of the American Civil War: Meagher serving with the Union, for which he recruited and commanded the Irish Brigade, and Mitchel allying himself with the South and living there, sending three sons to fight with the Confederacy.
The passage describes two individuals who fought on opposite sides of the American Civil War: Meagher, a Union supporter who led an Irish Brigade, and Mitchel, who supported the Confederacy and sent his sons to fight for the South.