Of course you can employ somebody in NI (or Spain, or Latvia -- wherever it suits).
What you need to get clear is where the employee's formal base is. If, as I would guess from what you say, the employee does all his or her work in NI and (another guess on my part) you pay in sterling, then you will need to register as an employer in NI and operate UK PAYE and National Insurance.
You can get an information pack from the UK authorities when you register, and you simply follow the steps. At first, it will be just another bloody nuisance, and then you will get used to it.