The Act of Union, passed in 1800, merged the Kingdom of Great Britain and the Kingdom of Ireland into the United Kingdom of Great Britain and Ireland. This legislative act abolished the Irish Parliament, effectively bringing Ireland under direct British control. In The Immortal Irishman, the Act of Union set the stage for political unrest and nationalist movements, including the efforts of Meagher and the Young Irelanders, who sought to repeal the Act and restore Irish self-governance.

The Battle of Antietam, fought on September 17, 1862, during the American Civil War, was one of the bloodiest single-day battles in American history, with significant casualties on both sides. The Immortal Irishman highlights the combat experiences at Antietam of the Irish Brigade, led by Meagher, showcasing their bravery and the heavy toll they endured. The battle's Pyrrhic victory for the Union forces influenced President Lincoln's decision to issue the Emancipation Proclamation.

The Emancipation Proclamation was an executive order issued by President Abraham Lincoln on January 1, 1863, during the American Civil War. It mandated that all enslaved people in Confederate-held territory were legally free. In the book, the proclamation is a turning point in the war; it affected public opinion and political dynamics in the Union.