He headed for America in 1535 enrolled in the expedition of Pedro de Mendoza and participated in the founding of Buenos Aires. He explored the Paraná and Paraguay Rivers along with Juan de Ayolas and was commanding the rear-guard when Ayolas's advance party were wiped out by the Payagua Indians.
Unique in Spanish America, the colony had been granted by Charles V the right to elect its own commander under such circumstances; and in August 1538, de Irala was elected by the conquistadors as Captain General of the Río de la Plata.
He outlasted the Charles V's appointment, Álvar Núñez Cabeza de Vaca, whom he had recalled to Spain for trial as a traitor. Although Juan de Sanabria and his son Diego were appointed governor in 1547 and 1549, they never fulfilled their commissions, and de Irala was confirmed by the king as governor in 1552.
He ruled forcefully until his death around 1556. During his rule, he had churches and public buildings erected, towns established, and the native population subjugated and distributed among the colonists in encomiendas. He was succeeded by Gonzalo de Mendoza.