The UK's impending Brexit has big implications for the border and the agreement that ended three decades of war.
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A good friend of Northern Ireland's, former US president Bill Clinton, is expected back today for the 20th anniversary of the Good Friday agreement.
It was a peace deal that ended a three-decade war that cost thousands of lives, that many said couldn't be done, and yet has become a model for conflict resolution around the world.
A Queen shook hands with former IRA commanders , and new cross-border arrangements meant Dublin took a place of influence as an EU partner shadowed and assisted by Brussels.
This is where a key problem for the future of the agreement and the peace it has bought lies, as the United Kingdom is on its way out of the EU, raising the prospect of new hard border.
This could explode the Good Friday Agreement. All parties agree that a hard border is not the way forward, but until now London's proposals for regulating the border post-Brexit have failed to gain support in either Belfast, Dublin, or Brussels.