You didn't give the month of the trip. It might be good to give a sketch of the itinerary, and the total number of nights you'll be sleeping in Europe/UK/Ireland. I would think a ferry ride could be more likely to be seasick inducing during the winter. Honestly, I think your time is too valuable to be spent investigating obsolete modes of travel. You need to understand that no one is being unsympathetic to you, but since there are two ferries and two trains between Dublin and Paris, you seem to have a romantic ideal that isn't very practical.
You might start with our host's general tips (blue menu top left.) But I looked and it's not immediately obvious from his hints that taking ferries between Dublin and Paris would not be a good idea. There are also issues of cost and bag handling, as well as your valuable vacation time. "Brexit" could also have possible bad effects on the plan. As it is, British immigration has always given my U.S.-born, 100% Americanski (Protestant, at that!) wife a hard time on arriving from Ireland because of her middle name. They are certain she is coming to take someone's job away.
Even if you want to consider it, you should investigate getting from your Dublin hotel to the ferry, and how many trains, and how much they will cost to get to London. Your OP Subject Line suggests that you aren't really planning to see London right now. Is that right? Honestly, with the uncertainty of Brexit, you might be better off leaving London for another time. But if you have particular objectives in England or London, my advice there is not worth much.
It is true that most train rides are more scenic than most corresponding car rides. But "scenic" is a relative term. I can tell you that the Eurostar train from London to Paris isn't very "scenic". We take trains all the time. But (see Rick's hints) the relaxing nature of the trip, and starting and ending in the middle of a city center is a big plus.