According to Correo del Camino it dates to the beginnings of the Camino de Santiago, but is specifically mentioned as a royal favorite during the 18th century and at one time was Spain´s most expensive cheese. By the late 1980s production had all but disappeared except for a few households in Pedrafita. Now, it has a Galician Denominación de Producto de Calidad. Completely natural - no preservatives or additives.
From the Castelo website:
The O Cebreiro, DOP cheese is a fresh cheese, without a noticeable rind, the body is white and granulated, soft, clayish, creamy, and melts in your mouth. It´s flavor and aroma are reminiscent of the milk from which it´s produced, slightly tart.
El Queso de O Cebreiro, DOP, es un queso fresco, no presenta corteza diferenciada, la masa es blanca y granulada, blanda, arcillosa al tacto, untuosa, fundente al paladar. Su sabor y aroma recuerdan al de la leche de la que proceden, ligeramente ácido.
More on production here.
If you can find some - I highly recommend it. Someday I have got to learn how to make this.