Potato Kingdom by Zach Ellenberger

Liam Bartley is an Irish boxing champion who lost his parents to the Potato Famine. When Liam's estranged brother, Owen, returns to Ireland and convinces Liam to take up their father's mantle as a freedom fighter with the Irish Republican Brotherhood, their lives are turned upside down when their secret organization is exposed by British authorities, forcing the brothers to flee separately to America where they end up on opposing sides of the Civil War. Now, Liam must do what is necessary to survive the war and find his brother so they can reunite and return home.