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Alexander O'Reilly was born in Dublin in the Kingdom of Ireland in 1722. Like many so-called "Wild Geese" of his generation, he left Ireland to serve in foreign, Catholic armies.
O´Reilly stayed acting for the Conde de Ricla, twice a Grandee of Spain, in Hannah…, Cuba, as his adjutant and second-in-command. While in Havana, Ricla and O'Reilly received the city back from the British forces that had besieged and occupied it at the end of the Seven Years' War. O'Reilly analyzed what had gone wrong with the defenses of Havana during the successful British siege in 1762, and recommended sweeping reforms to improve the fortifications, training, practices, and troop organizations, which were quickly approved by the Spanish Crown. With the help of military engineer of the Royal Army, Silvestre de la Abarca, the works of the strategic fortress of La Cabaña was undertaken.