Cork to Santander in October?

I can't help I'm afraid, but what a pity. Hard to see the reasoning. Getting the ferry from Dover to Calais there is quite a lot of restriction about which ferry sailings, and companies, will take a foot passenger, because of security I think - you have to go through lengthy series of checks while you are moved round on a bus, and in Dover you actually go through French immigration and customs. But you can do it. I wouldn't have thought Cork would be so complicated.

I do not know the ferry terminal in Cork but it might not have foot/bicycle access to the ferry itself. Foot passengers are catered for in Santander but not Bilbao because of the location of the latter port. Foot passengers have to go to Santander from Portsmouth.
Not sure if there is an easy answer for the OP