It seems that there is no direct option
From this quite old (2012) discussion on Le Routard.com (the website of a popular French tourist guide), there is no direct ferry route between Bretagne and Portugal. Ferries goes to Spain, in Gijon or Bilbao for example. This website confirms it was still the case in 2015.
This makes sense, as Portuguese main cities, namely Lisbon and Porto, are on the Atlantic Coast and reaching them would make the trip longer.
I found such a trip with Brittany Ferries:
Brittany Ferries vous conduit jusqu'en Espagne, à Bilbao ou Santander. Depuis ces ports, vous pouvez facilement rejoindre le Portugal avec votre voiture. Comptez environ 6 heures de route pour aller de Bilbao à Porto par exemple.
Brittany Ferries brings you to Spain, in Bilbao or Santander. From these ports, you can easily reach Portugal with your car. It takes about 6 hours of driving to go from Bilbao to Porto for example.
Also, their website mentions that it is possible to reach Spain through a short stop in UK:
Il existe d'autres possibilités via des lignes avec escale courte en Angleterre. C'est le cas de Roscoff - Santander via Plymouth, ou de Caen-Ouistreham / Le Havre / St Malo jusqu'à Santander ou Bilbao via Portsmouth. Il est également possible de faire une étape à Cork.
There exists other possibilities through lines with a short stop in England. It is the case of Roscoff - Santander via Plymouth, or Caen-Ouistreham / Le Havre / St Malo to Santander or Bilbao via Portsmouth. It is also possible to make a stop in Cork.
This is what I could gather using French websites. Since there is an important Portuguese community in France, I am pretty confident that if there was another option, I would have found it.