Corbyn will never win power, but he could still be disastrous for Ireland
Britain matters. Our nearest neighbour is, in so many ways, still our single most important neighbour. As such, the election of an extremely unusual politician as leader of the British Labour party should be a matter of importance and relevance on this side of the Irish Sea.
But in most ways it is less important than it might be. In only one very significant way is it particularly important.
Before discussing the one serious implication for Ireland, here's why Jeremy Corbyn's election matters much less than the hoopla would suggest: he won't be leader for very long and will never become prime minister. As he will never get to implement the policies he espouses, he is much less relevant than the British media's blanket coverage of his every utterance would suggest.
There are many reasons why he will quit or be ousted within a few years. Among the most important is that he will not be able to win over even a decent sized minority of his parliamentary Labour party - fewer than one in 10 Labour MPs supported him in the leadership campaign.
Leading a team of people, more than 90pc of whom wanted to be led by someone else, would be challenging even if the people involved were meek, forgiving and easily led. But professional politicians are not known for these characteristics. More often than not they are the very opposite: politicians are driven by ambition and ego and their tendency, even when things are going in their favour, is to scheme, manoeuvre and plot.
If there were no major divisions on policy matters, it would lessen (somewhat) the challenge Corbyn faces in trying to lead the overwhelming majority of the professional politicians in his parliamentary party who oppose him. But the policy divisions are huge.
Labour MPs are mostly of the centre-left. Corbyn is very far from the centre. He is instinctively statist and viscerally anti-market when it comes to the domestic economy. On foreign affairs, his starting point on most issues is anti-Americanism. These are both extreme positions which cannot easily be reconciled with those of the mainstream centre-left majority in his parliamentary party.
Add to these differences on policy the fact the Mr Corbyn will be in no position to appeal to the loyalty of his MPs. Over his decades in parliament, he never felt obliged to show loyalty to any of his five predecessors, voting against his party more than 500 times.
He did so because staying true to his many principals always trumped loyalty to his party. This may be admirable, if you believe compromise and flexibility amount to selling out. But no political party can function coherently if its people don't give and take on issues. Corbyn is likely to find that out the hard way very soon. And given his own track record, he will be in no position to demand loyalty when he adopts policies that most of his MPs oppose on principle.
All this will quickly lead to chaos. Conflicting signals will confuse and frighten voters. Infighting and ineffectiveness will alienate others. Labour's poll ratings will tank: later if Corbyn gets a novelty-factor bounce; sooner if he doesn't.
Because Britain's new fixed-term parliament system makes early general elections very hard to engineer, it is highly likely that the next vote will be in 2020, as scheduled. Corbyn will almost certainly be a distant memory by then. But even if, by some miracle, he were to survive for a full term, or if a premature general election is held, the chances of him winning power are minuscule.
The British people have shown little appetite for socialism of any kind, never mind of the Corbyn variety, over almost half a century. It is not a coincidence that six of the last seven Labour leaders never won an election, and the only one who did - Tony Blair - was also by a distance the least left wing.
While Corbyn may have won an overwhelming majority last weekend, the impressive-sounding 250,000 votes he garnered amount to less than 0.5pc of the entire British electorate. Anyone who believes that the views of this small, self-selecting group represents a sudden and sharp veering to the left by Britain as a whole is a fantasist.
So as there will never be a prime minister Corbyn, the implications for Ireland of a government led by him don't need to be considered.
That leaves the one big thing Corbyn could influence, and which could have a huge impact on Ireland - whether Britain stays in the EU. Over the next two years, the current Conservative government will hold a referendum on Britain's membership. If the vote is to leave, it will have overwhelmingly negative implications for Ireland, economically, with regards to Northern Ireland, on the Dublin-London bilateral relationship and on how the EU works.
With opinion polls this week showing a very small gap between those who favour staying in and those who want to leave, the referendum could go either way.
Before Corbyn came on the scene, the Labour party was poised to campaign in favour of remaining in Europe. Now that is far from assured.
Corbyn is a long-term Eurosceptic. The last time Britain had an in-out referendum on the EU - in 1975 - he voted to leave.
Just as the anti-EU right in Britain maintains the fantasy that a tree-hugging socialist sits behind every desk in Brussels by selectively focusing on the bloc's social and environmental agendas, the anti-EU left has difficulty seeing anything other than Europe's market liberalisation agenda. Ideologues are very good at seeing only those things which reinforce their prejudices, and from all of Corbyn's published remarks on the EU it is clear that he views it as a capitalist enterprise. Given this, it is hard to see him being a convincing campaigner in favour of staying in.
While he said yesterday that he could not foresee a situation in which he would back Brexit, his trade union supporters have intimated otherwise.
If David Cameron succeeds in getting opt-outs for Britain on some of the EU's social protection legislation, the unions have indicated that they would support leaving.
If Ukip and elements of the Conservatives are joined by Eurosceptic Labour party elements, pro-EU forces will have to fights populists on both flanks.
That could prove the Yes side's undoing.
With the election of Corbyn, the chances of Britain leaving the EU have just risen. That is not good for Ireland.