Cheap way to go from Paris to either Belfast or Dublin, without flying?

Just get the plane, it will only take an hour and a half. The 2 ferry crossings and 3 train journeys will take over a day and a half. Don't worry about the carbon footprint, the plane will be flying to Dublin anyway, whether you are on it or not. So the fact that you have left an empty seat will make no difference. Hire a bicycle when you get here and cycle to Belfast, that should ease your conscience.