The agri-food sector – trade in food and farm produce – dominates trade in goods between the north and south.
A 2016 UK parliamentary report, external, citing the Irish Farmers’ Association, says that 350,000 lambs went from north to south, 500,000 pigs from south to north, and millions of litres of milk travelled in both directions over the course of a year.
Almost 30% of Northern Ireland’s milk is processed in the Republic of Ireland, where it is made into butter and cheese or used in infant formula.
For members of the EU, goods and produce such as these can cross borders freely.
But if negotiations between the UK and the EU fail to reach a trade agreement, or if the agreement is less favourable than that enjoyed by member states, it could mean that tariffs – taxes imposed on imported items – could become costly.