Not sure where to list Belfast ~ under UK? under Ireland? We use Lonely Planet guides on most of our travels and they list Northern Ireland in the Ireland book. I’ll play it safe and list it under all three: Ireland, UK, and Northern Ireland. Belfast was not on my radar as far as planning this trip but John insisted we go for at least one night. He wanted to make the boat trip into Ireland because some of my family are from Ireland and they immigrated to the US/Canada in the mid-1800s. John said “They left by boat, we should return by boat.”
Stena Line ~ Loading for Belfast
Luxurious Seating on the Stena Line
Ferry Boat to Belfast: Stena Line
We took the train from Glasgow to Ayr (about an hour) then a bus from Ayr to Cairnryan (about 90 minutes) then a ferry from Cairnryan to Belfast (about 2 hours). Stena Line is the nicest ferry I’ve ever been on ~ downright luxurious and we were in steerage. I’m used to bench seats and small ferrys. It was a full day of travel but interesting to interact with those making their way to Belfast for the Easter Weekend.
Displaced Beachbums 
Let me know what you think...