Brexit part 2/5: The great Irish divide
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Violence between Republican and Unionist groups in Northern Ireland left more than 3,500 people dead from the 1950s until the Good Friday Agreement peace deal was signed in 1998.
That agreement phased out security checkpoints on the border between Northern Ireland in the UK and the Republic of Ireland in the south and allowed people and goods to cross freely and unchecked.
This was helped by the two countries being in the same customs union and single market, removing any need for inspections at the border. In March 2019, Northern Ireland will leave the EU and the Republic will stay.
So how will the border be policed? And does this risk a return to the violence of the past?
Andrew Connelly has this report from Northern Ireland.