it seems there is still a problem booking with a bicycle with Brittany Ferries from Ireland to Spain/France but not from the UK, I tried some random dates in September from Rosslare to Bilbao and although it's possible to make a booking on this route with a vehicle if 'bicycle' is selected instead then 'no sailings' comes up on the online booking form
I haven't tried it for a couple of years but it seems that there has often been an issue taking a bicycle to Bilbao. TheManinSeat61 and I had a slight disagreement over it. He maintained it was perfectly possible but trying to book from Portsmouth wasn't easy. Apparently you can if you telephone rather than book on line. Of course this was long before COVID restrictions.