Northern Ireland is part of the United Kingdom.
You will need a Electronic travel authorisation (ETA) to enter.
Although that is a sore point for many people who live there (whilst many others completely prefer to remain in the United Kingdom) the laws of of the UK are applicable once you enter any of the 6 counties of Northern Ireland. The £GBP pound is the official currency and the road signs use miles and not kilometres.
So you need to apply for a United Kingdom Electronic travel authorisation (ETA).
The process is very simple and you simply apply olive by following the guidelines in this link provided by the UK authorities:
https://www.gov.uk/guidance/apply-for-an-electronic-travel-authorisation-eta
