And they’re gone. After three-and-a-half years of tortuous negotiations, political chaos and bitter social upheaval, the United Kingdom will formally leave the European Union at 11pm on Friday, signalling retreat from its most important alliance and casting itself into uncharted waters filled with hazards it must now contend with alone. For Brexiteers, the realisation of their political project is a time to rejoice. But for Ireland, the EU and the millions of British citizens, including so many young people, who cherish their European identity, this is above all a profoundly sad moment.

No state in the modern era has committed such a senseless act of self-harm. Brexit will make Britain poorer; the British government’s own analysis predicts as much. But the real impoverishment is far broader. Its citizens’ freedoms will be curtailed. Its voice in the international arena will be weakened. Its reputation as an open, forward-looking country will be diminished.

The Brexiteers’ argument all along has been that to leave the EU was to choose control over influence. But Britain’s influence was real – London shaped the course of European integration more than most, and the EU amplified its voice in the world – but the control it claims to regain on Friday night is fantasy.

The EU will continue to be its biggest trading partner and home to its closest allies. As a European state, it will be affected in one way or another by EU policies in almost every key area. But Britain will have no say over these decisions. Instead it has opted to luxuriate in the bogus comforts of a radical English nationalism that promises to fend off the forces that shape the modern world while retreating to an imagined past. Meanwhile, with Northern Ireland and Scotland being dragged out against their will, the multinational experiment that suddenly looks liable to implode is not the EU but the UK itself.

For Ireland, a Brexit negotiation process in which it had more power than its neighbour has been a useful reminder of the EU’s genius. The continent’s solidarity with Ireland over the past three years will not be forgotten, and our place at the heart of the EU is secure. But for the EU and for Ireland, Brexit is, inescapably, a terrible loss. The EU will be weaker without one of Europe’s traditional powers. And Ireland has lost a valuable ally at the EU table.

There are two kinds of EU states, a recent Danish foreign minister observed: small states and those who do not yet realise they are small states. On Friday night, two small states that signed up to the European project on the same day in January 1973 will go their separate ways, having come to very different conclusions about the values they stand for and their own place in the world. We may be growing apart, but our friendship can – must – endure.