Boris Johnson says he wants Britain out of the European Union. Not much of a story, that. Mayor Boris has been a Eurosceptic for many a year. But the timing of his intervention in the debate is interesting.
He has followed two former "big beasts" of the Conservative Party, Michael Portillo and Lord Lawson, among the notables who intend to vote against British membership of the EU in the in-out referendum in 2017. Their ranks will swell further as time goes by.
The word 'former' points up a big difference. Mr Portillo and Lord Lawson are yesterday's men. Mayor Boris is very much today's man, and bent on becoming tomorrow's man.
And nobody knows that better than David Cameron. At every twist of the story, the prime minister can feel the breath of the Mayor of London on his neck.
Boris wants his job, and for all the mayor's clownish ways, he must have a good chance of getting it sooner or later.
That makes him different not only from the Portillos and Lawsons but from a practising politician whom we must now take seriously.
It is massively unlikely that Nigel Farage will ever be prime minister. But he and his United Kingdom Independence Party can do dreadful damage to the Tories. They have already done so, with the sweeping successes they scored at the expense of the Conservative Party in the recent local elections.
How does a leader already weakened by his fraught relationship with his coalition partners, the Liberal Democrats, respond? All experience suggests that the reaction of Mr Cameron and his government will be to move farther to the right.
He has held the line on Europe by saying that he will not recommend a referendum vote in favour of staying in the EU without substantial concessions. But meaningful concessions are close to impossible. Such little credibility as he now enjoys on the subject can hardly last until the 2015 general election, much less all the way to 2017.
We therefore have to accept that British withdrawal from the European Union has become a live possibility, and consider the implications for Ireland.
These are nothing less than scary.
One of our great success stories – and yes, we do have success stories – has been the diversification, along with the growth, of our exports. But the UK still accounts for 16pc of these exports. Imagine Ireland, with an overpriced currency, trying to maintain our trade with a major trading partner which, outside the EU, would probably devalue sterling.
The Northern Ireland Border could become a problem again. Bad enough that we trade too little with the North, and bad enough that we have different currencies; worse if the trade were subject to a new agreement in which the UK would dictate the terms.
And this would be only a small part of the general economic and political disruption which Britain's departure from the EU would cause for the entire union.
As matters stand, we have little hope of influencing the British debate. Our best hope lies in the certainty that the UK business and financial world will move every muscle in the campaign against withdrawal.
On the party-political side, British Labour this time will surely not revive its bad old habit of making foolish decisions. A pity that the party leader, Ed Miliband, is so unimpressive, but he could hardly be much less effective than David Cameron.
And he has an excellent chance of taking power in 2015, in a Labour government or in coalition with the Liberal Democrats.
Must we, then, accept that much of our future may depend on the vagaries of the British electoral system? To a great extent, the answer is yes. But that does not exempt our Government from making contingency plans. Politicians love to leave things until the last minute.
This time, the potential consequences are so huge that they would be wise to start making the contingency plans right away.