Three options for controlling border following Brexit
A 'hard' border between the Republic and Northern Ireland - This is the option both Governments claim to want to avoid.
Brexiteers have talked up the need to "take back control of our borders", so it remains to be seen how the UK government can stand true to this commitment, while leaving its land border with the EU open.
Enhanced border controls at Irish ports and airports
Ireland is not in the Schengen area, which allows for passport free travel among certain member states. That means that if you're travelling to Ireland from another EU country, you have to go through an immigration control point anyway. The two governments already work closely together to boost the Common Travel Area border. Such a move could be politically difficult, as Ireland could be seen to be taking on the role of UK border control.
A border between the island of Ireland and Britain
That could see travellers from, say, Belfast to Liverpool having to go through border checks - a move likely greeted with hostility from unionists.