British and Irish citizens (but not British "nationals" from Hong Kong or other places overseas) can fly within the British Isles with a photocard driving licence.
Other EU citizens can use their national ID card.
Everyone else still needs a passport.
If you are travelling from the USA to Northern Ireland, you need a passport to go there. You probably will also need a visa to enter Northern Ireland but you can find this out if you look up the British government website.
That will depend on your nationality. Irish and British citizens can travel between Ireland and Britain without needing a passport, though many will bring their passport with them anyway. If you are not from Ireland or Britain, then you will need a passport, and may even need a visa.
yes there are no border restrictions from the republic,
As Belfast is in the United Kingdom and Dublin is in the Republic of Ireland, technically you may need a visa or passport to go there, depending on your nationality or other factors. However, there are no passport checks done on the train. Usually if you have been allowed access to Northern Ireland, you will also be allowed to get into the Republic of Ireland, but you need to check. See the website at the link below.
No. You don't need a passport to go to any part of Ireland from anywhere in the UK. However, due to increased security in recent years, you can't board an aircraft without an acceptable form of photo ID and a current passport is the best form of ID. If you are travelling by ferry, you won't need a passport, but it is advisable to check with whichever carrier you are planning to use as the rules could change.
yes, because the republic of ireland is a seperate country from the UK you will need a passport to go there.
Northern Ireland is part of the UK. A citizen would not necessarily need a passport. Someone from outside of the UK would need one. As a general rule, a person should always have a passport when flying anywhere in the world.
That will depend on what your nationality is. You would need to check with your local authorities or with the Irish department of Foreign Affairs. Check their requirements in the link below. If you plan to go to Northern Ireland, then the requirements could be different.
It depends where from.
none if you are European. There are no border posts
depends where your coming from, see the related link.
NoYou will need a passport or identification depending on your citizenship.