The bureaucratic elements involved for a move to Northern Ireland are, as far as I'm aware, the same as for moving to anywhere in the UK. The UK-Yankee
may be a good place to start; it gives an overview of the process from both a practical and technical point of view. You can also go directly to the source; the Visa and Immigration department
can offer non-specific (as in they can explain the process and what one needs to do but can't give tailored advice).
I'm not an immigration specialist so take this with a grain of salt but from what I understand she would first have to apply for leave to enter the UK and then, once that has been accepted and she is in the UK, would have to apply for leave to remain. As I understand it the key to getting a visa for a long term stay is that the immigrant can show they would not be a burden on the state. In this case and assuming she doesn't have a job lined up where the employer is willing to sponsor her that would mean her partner has to demonstrate his ability to support her between his income and savings. I think that requires an income of at least £18600 a year, but double check.