The UK and Ireland operate a "common travel area" This means that legally you do not need a passport. However a passport is now the only form of identification accepted by most airlines operating between the UK and Ireland.
only if you are going to Northern Ireland, you need a passport to travel between England and Ireland.
No. There is a common travel agreement between Ireland and Britain. You will not need a passport at any time when travelling across the sea between Britain and Ireland. Neither do you need one when travelling in Ireland, from the south of the country to the North. There is no border on the island. There is open travel between the islands/countries of Ireland and Britain. Google 'Common Travel Area Britain Ireland' .
No. There is a common travel agreement between Ireland and Britain.
You don't need a passport to travel between the UK and the Republic of Ireland.
No, you do not need any vaccines when you travel to Ireland.
There is not any direct rail connection between Belfast and France. You will need to travel via London, using ferry service. It will cost you $90 to travel from Belfast to London (ferry and train both), and another $117 to travel to Paris from London via train.
Yes
Yes.
From where?
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.
No
yes