depends on the airline and where you are flying from. Some require nothing (BA within the UK) some need "something offical" and some need a passport. Several airlines don't allow kids to fly without their parents.
Different airlines have different rules, so check with the airline that you intend to travel with.
Same as adults, but check with the airline that you intend to travel with.
Yes they do need a passport for Northern Ireland
People should bring Euros to Ireland, as that is the currency that is used in the Republic of Ireland. If they are going to Northern Ireland too, then they will need Sterling to spend there.
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.
It should be fine as it is. As long as your initial marriage is legal, there shouldn't be any need to register it in Northern Ireland. It will be recognised.
No. Northern Ireland is part of the UK, so the visa will cover it.
It depends on where you are coming from As Northern Ireland is part of the UK you will need the same Visa that you would need to enter any other part of the UK. If you are a UK citizen you do not need one
No.
No! Don't be thick! And there isn't any need to write "the" before Northern Ireland.
They will need a visa to enter the United Kingdom in order to enter Northern Ireland.
only if you are going to Northern Ireland, you need a passport to travel between England and Ireland.
No.No.No.No.No.No.No.No.No.No.No.
none if you are European. There are no border posts