Travel from Ireland to northern Ireland?

Hi,

I have a friend coming to Ireland from USA in June, she will be staying in Ireland for about 3 months then is intending to spend a further 6 months in Northern Ireland.

Will she need to apply for an Irish visitors visa before coming to Ireland?

Will she then need to apply for a visa for the UK when she comes to Northern Ireland?

If so, where does she apply for the visa before coming to Northern Ireland? And is it possible to get a 6 month visitor visa for the UK?

Any information is appreciated, Thanks.

US citizen? No visas required for visiting purposes.