 He was commissioned by Governor Juan Manuel de Rosas to settle a boundary dispute with Chile (Rosas was charged with the Argentine Confederation's foreign policy during his 1835-52 reign), and from 1844 to 1850, Irigoyen served as Justice Minister in Mendoza Province, where he enacted the first provincial judicial system, as well as reformist military law and land law statutes.He again negotiated with Chile over the disputed Straits of Magellan (1851), and following Rosas' overthrow, helped draft the 1852 San Nicolás Agreement.He participated in the constitutional assembly that paved the way for the 1860 reunification with secessionist Buenos Aires Province, and was nominated to the Argentine Supreme Court; he refused, however, and resumed his private practice.