De Lacy is born to Lieutenant-Colonel Patrick de Lacy, an officer in the Ultonia or Ulster Regiment, a foreign unit or Infantería de línea extranjera of the Spanish army. Patrick dies sometime before 1785 and his wife, Antonia, remarries Jean Gautier, another Ultonia officer. His grandfather, General Patrick de Lacy y Gould, came from Limerick. Along with many relatives, he was part of the post-1691 Irish diaspora known as the Flight of the Wild Geese.
De Lacy is commissioned into the Ultonia regiment when he is ten, although his age is recorded as thirteen to satisfy minimum requirements. Issuing commissions to children is not unusual, as they are considered private investments and often used to provide pensions for orphans. Although by now the Ultonia is no longer “Irish,” many of the officers are Spanish-born descendants of the original Irish emigrants, including his uncle Francis and various cousins.
In 1789, de Lacy joins an expedition to Puerto Rico, accompanied by his stepfather. They apparently quarrel and on their return, de Lacy walks to Porto, in Portugal, intending to take ship to the Maluku Islands, before his stepfather brings him home.
Promoted captain, de Lacy takes part in the War of the Pyrenees against France, which ends with the April 1795 Peace of Basel. He is posted to the Canary Islands in 1799, where he fights a duel with the local Capitán-General. Despite being transferred to El Hierro, he continues their feud. He is court-martialed as a result and sentenced to one year in the Royal Prison at the Concepción Arsenal at Cádiz.
De Lacy’s jailers allegedly consider him mentally unbalanced. As a result, he is stripped of his commission and barred from re-enlisting in the Spanish army. He moves to France in order to continue his career and is appointed captain in the Irish Legion, a French army unit formed in Brittany and intended to support an Irish rising. Although many of its officers are Irish exiles or of Irish descent, the rank and file are mostly Polish.
When the proposed rebellion fails to materialise, the Legion is posted to the Netherlands, where it remains until the War of the Third Coalition ends in 1806. De Lacy is appointed commandant of the second battalion, which participates in the 1807 Invasion of Portugal. In March 1808, Charles IV of Spain abdicates in favour of his son, Ferdinand, who is replaced in May by Joseph Bonaparte and held in France.
In July 1809, de Lacy is given command of the Isla de León, an important defensive position in Cádiz, home of the Regency Council that rules Spain in Ferdinand’s absence. He leads the 1st Division at the Battle of Ocaña on November 19, 1809. The collapse of the Spanish cavalry under Manuel Freire de Andrade exposes him to a flank attack that practically annihilates his division. A second defeat at Alba de Tormes on November 29 leaves the Spanish unable to confront the French in open battle and they resort to guerrilla tactics.
Although Cádiz is besieged by the French from February 1810 to August 1812, support from the Royal Navy allows the Council to send small amphibious expeditions intended to bolster resistance elsewhere. De Lacy leads landings in Algeciras, Ronda, Marbella and Huelva and although unable to hold them, this absorbs French resources. In March 1811, his troops support an Anglo-Spanish attempt to break the siege of Cádiz. The resulting Battle of Barrosa is a significant victory, although command failures mean the siege continues.
After the loss of Tarragona in June 1811, de Lacy replaces the Marquess of Campoverde as Capitán-General of Catalonia, a position held by his uncle Francis from 1789 to 1792. French efforts to capture Valencia weaken them elsewhere and provide the Spanish opportunities for partisan warfare. He leads a series of incursions into the French departments of Haute-Garonne and Ariège. These restore local morale and force the French to send reinforcements.
Most major towns, including Barcelona, Tarragona and Lleida, remain in French hands and in early 1812, Napoleon makes Catalonia part of France. The focus on guerrilla tactics lead to an increasingly bitter war of reprisals and executions by both sides, which severely impact the civilian population. Many of the partisan bands are beyond central control and their operations often indistinguishable from simple brigandage. This leads to conflict between de Lacy and local Catalan leaders and in January 1813, he moves to Santiago de Compostela as Captain General of the Kingdom of Galicia. He assumes command of the Reserva de Galicia, which he focuses on disciplining and reorganising. Following Allied victory at Vitoria in June 1813, the French withdraw from Spain and Ferdinand returns to Madrid in April 1814.
Ferdinand rejects a previous commitment to accept the Spanish Constitution of 1812 and establishes an absolutist regime. Spain also faces colonial wars in the Americas, which begin in 1810 and continue until 1833. This destabilises the regime and leads to a series of attempted coups, by military officers like de Lacy backed by progressive civilian elements, often linked by Freemasonry.
Following failed attempts in 1815 and 1816, de Lacy returns to Barcelona and assisted by a former subordinate, Francisco Milans del Bosch, plan another. This begins on April 5, 1817 but quickly collapses. De Lacy is captured, court-martialed, and sentenced to death. Following public protests against the sentence, he is secretly taken to Palma de Mallorca, held at Bellver Castle and executed there by firing squad on July 5, 1817.
In 1820, a revolt led by Colonel Rafael del Riego forces Ferdinand to restore the 1812 Constitution. This begins the Trienio Liberal, a period of liberalisation that ends in 1823, when a French army allows Ferdinand to re-assert control. However, in 1820 the reconstituted Cortes Generales declares de Lacy a martyr. Along with others including Riego, he is commemorated on a plaque in the Palacio de las Cortes, Madrid, which can still be seen today. De Lacy is buried at the Cementiri de Sant Andreu, in Barcelona.