Northern Ireland is part of the United Kingdom and also part of the Common Travel Area of Great Britain, Northern Ireland and the Republic of Ireland.
UK passport holders can of course enter Northern Ireland freely but note that airlines may require a photo ID.
EEA (the EU, plus Norway, Iceland and Liechtenstein) and Swiss citizens can travel to the UK for holidays or short trips without needing a visa. However, you will need to show a passport if first arriving in England, Scotland or Wales before travelling on to Northern Ireland. However, you will not be asked to show identification if you are entering Northern Ireland from the Republic of Ireland.
Useful links:
Check if you need a UK Visa – https://www.gov.uk/check-uk-visa/y
Travelling to the UK from Ireland – https://www.gov.uk/guidance/travelling-between-the-uk-and-ireland-isle-of-man-guernsey-or-jersey