Ferreiro was born in a well-off Galicianist family. In 1932, at the age of twenty, he created the Mocedades Galeguistas de Celanova (Youth Galicianists of Celanova) with Xosé Velo Mosquera. In 1934 he also participated in the creation of the Federación de Mocedades Galeguistas (Federation of Youth Galicianists). Some time after this Ferreiro got into trouble because of an article published in his magazine Guieiro.
In 1966 Ferreiro travelled to Venezuela, where he collaborated with the Galician Brotherhood. He fell out with the Galician nationalists in Venezuela, and in response published the poetry collection Viaxe ao pais dos ananos (Journey to the land of the dwarves). He founded the Patronato da Cultura Galega (Patronage of Galician Culture), and was part of President Rafael Caldera's cabinet.
After returning to Spain Ferreiro lived in Madrid, where he worked as a journalist. He wrote in Galician and in Castilian, but his most important work was his Galician-language poetry. "Longa noite de pedra" ("Long night of stone"), a lament at the anti-Galician policies of the Franco regime, is one of the best-known and most powerful Galician poems of all time.