Irish ferries does that route and it is seasonal.

When are you travelling

This year has finished and the 2019 timetable is not yet published

Fly.

We have taken both Irish Ferries and Brittany Ferries to Roscoff on a number of occasions - most recently last June. And it will take you between 14 and 18 hours to sail, plus a further 8 hours drive to Bordeaux (Yes, we have driven that route too). And you also need to add embarking and disembarking time.

I am not sure but the Cork Santander ferry may get you closer to Bordeaux, even if it is a longer sailing time.

With a car, Santander would be better and the Friday evening departure time may suit better. But I assume we are talking about a foot passenger here and that ferry doesn't take foot passengers.

Aside from the Ryanair flights from Dublin and Cork I mentioned previously, it looks like Aer Lingus fly from Dublin and Easyjet fly from Belfast, so there is quite a bit of choice for flying. It does look like the best solution.

I cannot see any reason for anyone who does not have their own car to take a ferry from Ireland to France, especially when the ferry does not drop you anywhere near the destination. It takes much longer and is more expensive.

For someone with their own car, travelling by ferry is very practical as you can transport loads of things, which might be very advantageous if planning a week or 2 self catering in a cottage or family camp in France. It also makes financial sense as with a family you might want a car, so bringing your own might cost more for getting to France but then saves you on car hire. But seeing as you are flying in from abroad, this doesnt seem likely, nor have you the need for a car full of equipment seeing as youre only getting a river cruise.

If you plan to hire a car in Ireland and drop it in France, have you even checked if that's possible, either to have the car on a ferry or drop it in France (funnily, there's no demand for cars with the steering wheel on the wrong side of the car in France) .

Summer schedules for the airlines and ferries are coming out slowly these past few and coming few weeks, so if you need more options just give it another 2 to 3 weeks.