O'Brien, John Thomond (1786–1861), bullfighter and military officer in Argentina, was born 24 June 1786 at Baltinglass, Co. Wicklow, son of Martin O'Brien, a wealthy farmer, and Honoria O'Brien (née O'Connor). In 1811 he emigrated to South America, settling first in Brazil. His attempts to found a number of business ventures were unsuccessful and in 1812 he moved to Argentina, but his luck was no better at Buenos Aires. Turning to a life of adventure, he joined the army and fought with distinction at the battle of Montevideo (1813). An accomplished bullfighter, he was greatly admired for his bravery and technique; in 1816 he astonished the crowd by somersaulting over the charging bull.
After briefly resuming his civilian life, he found the call for volunteers impossible to resist and joined José de San Martín, the Argentine general who would go on to liberate South America from Spanish rule. After patrolling the Portillo Pass in the Andes, he was appointed aide to San Martín, becoming his ‘inseparable adjutant’ (Murray, 43), and accompanied him on his campaigns in Chile and Peru. Pursuing the Spanish general, Osorio, after the decisive battle of Maipú (1818), O'Brien succeeded in capturing all the staff papers and a number of important prisoners. He returned to Buenos Aires after the battle of Callao in Peru and was given the notable honour of bearing all of San Martín's captured trophies and banners. He later went to Bolivia, where he received a silver mine as a reward for his military expertise. Closely monitoring events back in Ireland, he formed a group with other Irish émigrés in 1828 to discuss how to help the movement for catholic emancipation.
In 1834 he retired from the Argentine army and set out to explore the Amazon, settling for a time in Uruguay. Appointed consul general to the UK by President Rivera of Uruguay, he returned to Europe in 1854. He died 1 June 1861 in Lisbon and was buried there. He married Rebecca (surname unknown), and left two daughters. In 1935 his remains were brought back to Argentina where he was buried as a national hero.