I did a similar trip on the Caribbean Princess in May of '17, but two days were in Dublin rather than Belfast. It was a nice taste of Ireland, Scotland and France. I want to get back and have been looking at land vs. cruise. I'm not sure I want to pack and unpack so many times for hotels and I wonder how much of the trip will be in a bus.
Princess is my favorite line, but for the right itinerary I will look elsewhere. NCL has an entire Ireland cruise on May 8th of next year. It hits the Irish ports you list above and Waterford, Dingle, Galway and Killybegs.