Belfast is the capital of and largest city in Northern Ireland. Northern Ireland is one of the four countries of the United Kingdom.
I can’t see why but read
No but you do need I.D.
Immigration and visa requirements
Northern Ireland is a constituent part of the United Kingdom, meaning it has exactly the same entry requirements as England, Scotland and Wales. Citizens of the UK and Crown Dependencies can travel to Northern Ireland without a passport and have the automatic right to reside and work.
Citizens of the European Union do not require a visa, and have permanent residency and working rights in the UK.
Check with the ferry operators
Best thing to do is bring your passport it saves a lot of hassle, also when going over the border they will check your passport, also change your money from £ to euro as they use euro in the south, also change any Northern money back to Bank of England notes as you will have problems when you go back home.
Have a nice trip.