But Constanza's husband, Infante Peter, fell in love with the young Inês, and carried on openly with her, much to the scandal of the Portuguese court.

The influence of Álvaro and other exiled Galician-Castilian nobles upon Infante Peter alarmed Afonso IV, who feared they might drag Portugal into the internal quarrels of neighboring Castile, part of which were orchestrated by their half-brother Fernando Ruiz de Castro.

With the resolution of 1371, Álvaro Pires de Castro was settled by Ferdinand I in Portugal with the title of Count of Viana (da Foz do Lima).

In 1382, Ferdinand I created the position of Constable of Portugal (leader of the Portuguese nobility) and named Álvaro Pires de Castro as its first holder.

Álvaro Pires de Castro died in July, 1384, and he was buried in a tomb alongside his wife in the monastery of São Domingos of Lisbon.