Since Northern Ireland is part of the UK, I expected an ID check, but there was no such thing on arrival in the train station. They do control access to the platform though, and the seat displays show the name of the booked passenger. The Irish were always applying EU privacy regulations loosely.
Belfast has a great Titanic museum at the site where the Titanic was built and launched. They even have a amusement park style ride that goes up and down in the building to show the stages of construction. Gives a really good impression of the size of the ship.
Of course Belfast has the usual cathedrals and marble city hall and shopping streets, and it feels a lot like Dublin, but I think I like Dublin more. It feels more accessible and walkable. Also cheaper – the return trip by train, purchased in Belfast, costs more than twice aa much.