From the Cantabrian:

When you're in Galicia, once you've passed Tapia de Casariego, drive in direction to Lugo. From Lugo take the N-640 up to Lalín, where you take the PO-200 until you can connect with the N-541 in direction to Pontevedra. Once you're in Pontevedra, you have to take the PO-12 up to Marín for 8 km.