These tomb like structures are known as espigueiros. There is actually nothing macabre about their purpose and the are in fact grain stores.
They are only found in the northern border regions of Portugal but I have also seen them in Galicia in Spain. These particular espigueiros are located in the village of Soajo in the Peneda-Gerês National Park.
Generally made of granite these granaries are designed to keep the corn or grain at the right humidity. This is one reason they are raised off the ground on pillars; the other is to keep the grain safe from rodents and errant hens.