Yes, there is free movement between the two countries
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.
Unless you are a citizen of a country that has a visa-free arrangement with Ireland, you cannot enter the country without one.
They will need a visa to enter the United Kingdom in order to enter Northern Ireland.
No. You need a visa to go to England as a Malawian citizen.
Ireland
Northern Ireland is part of the UK so your UK visa is valid.
yes there are no borders between northern and southern ireland
Yes.
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, as long as you are a passport holder of England or USA, you can fly in between without a visa
Yes
Fijian citizens can visit Ireland visa-free for 3 months.