Northern Ireland is part of the UK so your UK visa is valid.
No.No.No.No.No.No.No.No.No.No.No.
As Northern Ireland is still part of the UK; the same rules will apply.
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
Yes South African passport holders require a visa. You cannot get an Electronic Travel Authority (ETA). You must apply in person at your nearest embassy.
Yes.
yes there are no borders between northern and southern ireland
No
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 !
Yes, the visa allows free travel in the entire United Kingdom (the UK) of Great Britain and Northern Ireland territory.
Ireland is not part of the Schengen Area so Schengen visas are not valid in Ireland (or the UK). You may need a visa unless you hold a passport from a country which doesn't need a visa for Ireland (e.g. USA). Check the Irish embassy for your country to see their rules.