Yes, Jamaicans need a visa to enter Ireland.
yes i need visa to enter Ireland
They will need a visa to enter the United Kingdom in order to enter Northern Ireland.
Yes.
No, Serbian citizens doesn't need visa for Schengen countries, but they need visa for UK and Ireland.
No - you still need a visa to enter the UK !
Usually a separate visa is required to enter England than to enter Ireland, so depending on your own circumstances, if you need a visa to enter Ireland then you will also need another one to enter England. There are proposals from the Irish government to let people who have visas for Britain to enter Ireland without having to get another one for Ireland. As of the time of answering this question (June 2011) this has not been put into place. Even when it does, they doesn't mean that a visa to Ireland will be acceptable to England.
I think so.
No. Northern Ireland is part of the UK, so the visa will cover it.
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.
Unless you are a citizen of a country that has a visa-free arrangement with Ireland, you cannot enter the country without one.
That will depend on what country you are from. British and Irish citizens don't need one to enter Ireland. Citizens of other countries would need one, and some may even need a visa to get to Ireland.