Northern Ireland is unique as it shares a land border with another EU country – the Republic of Ireland. If the UK leaves the EU, this border would become a border between an EU country (Republic of Ireland) and non-EU country (UK).
There would be little or no change to current arrangements. UK and ROI have a Common Travel Area which has been in existence long before the EU.
There can be no guarantee that the current arrangements would continue as there may be immigration checks (of people) or customs checks (of goods) along the border between NI and ROI.