The 'common travel area' and its role in Northern Ireland
On 23 June, the UK is having a referendum on whether to remain in or leave the European Union.
With Northern Ireland being the only part of the UK with a land border with another EU member, the impact of the decision would be far reaching.
BBC Northern Ireland Economics and Business Editor John Campbell looks at what is known as the 'common travel area' and its role between Northern Ireland and the Republic of Ireland.