Aspirations of social mobility and anti-Catholic discrimination were the lifeblood of subversive opposition to British rule in Ireland during the mid-nineteenth century. Refugees of the Great Famine who congregated in ethnic enclaves in North America and the United Kingdom supported the militant Fenian Brotherhood and its Dublin-based counterpart, the Irish Republican Brotherhood (IRB), in hopes of one day returning to an independent homeland. Despite lackluster leadership, the movement was briefly a credible security threat which impacted the history of nations on both sides of the Atlantic....