Alejandro O'Reilly, 1st Count of O'Reilly, KOA, English: Alexander, Count of O'Reilly, Irish: Alastar Ó Raghallaigh, was an Irish-born military reformer and Inspector-General of Infantry for the Spanish Empire in the second half of the 18th century. O'Reilly served as the second Spanish governor of colonial Louisiana, and is the first Spanish official to exercise power in the Louisiana territory after France ceded it to Spain following defeat by Great Britain in the Seven Years' War. For his much appreciated services to the Crown of Spain, O'Reilly was ennobled as a conde de O'Reilly, and granted a coat of arms. He fell out of favour after his calamitous failed attempt at the Invasion of Algiers.

Brigadier Juan Mackenna was an Irish-born, Chilean military officer and hero of the Chilean War of Independence. He is considered to have been the creator of the Corps of Military Engineers of the Chilean Army.