re: Edinburgh

Looking at the Stena Line site, you can't take a ferry from Dublin to Cairnryan (coastal town in Scotland), you can only ferry from Dublin to Holyhead in Wales.

So you'd have to take a train to Belfast (2 hours), hop on the ferry(2.5 hours travel time, but need time for loading/unloading), get on a bus to basically Glasgow(2 hours) and rent a car to drive to Edinburgh (a little over a 1 hour drive plus getting the car time).

You're better off flying both ways if you must see Edinburgh. We only spent a day there and it was cool to see, but lots of tourists. I enjoyed Scottish highlands and isles (Oban, Skye, Inverness is where we stayed overnights) significantly more than Belfast or Edinburgh. But a two day jaunt to the highlands would be a whirlwind mess itself.