Travel from Ireland to France

I can't speak for the level of comfort on the boats, but you can choose different ports and compare prices and schedules on this consolidator site, which handles multiple companies.

https://www.aferry.com/ferries-to-france.htm

Thanks to Ryanair and its competitors, Ireland is well stocked with very cheap plane tickets to anywhere In Western Europe, or so it seems. However comfortable a ferry, rough seas in those channels are not unknown. So is airline turbulence, I suppose, but the flights don't last more than 90 minutes or so. Bought far in advance, a ticket can be $50 to $100 US.
www.skyscanner.com