After Enda Kenny was re-elected as Taoiseach, British prime minister David Cameron phoned him. The two men have a warm relationship, and Cameron's congratulations were genuine.

But that was not the only point to the call. Cameron has staked his political future on winning the British referendum on EU membership, on June 23rd. In his bid to keep Britain in the union, he needs help from whenever he can get it, and the Taoiseach promised he would do all he could.

Mr Kenny later told the Dáil that the British referendum campaign is “entering a critical phase”.

“I strongly believe it is critically important that our voice is heard, as Britain’s closest neighbour and indeed friend, on this issue,” Mr Kenny said.

Ireland’s most important relationship is with the UK and relations between senior politicians and civil servants, say people on both sides, have never been better.

But Ireland's interest in the referendum is not entirely altruistic – extensive analysis conducted by Irish officials since Cameron first made his pledge of an "In-Out" referendum has concluded that Brexit poses a major threat.

Much of that threat is unquantifiable but it would certainly present security, commercial and political difficulties for Ireland.

If the British decide to leave, said one senior official, it would dominate the Government’s term of office in the way that the financial crisis dominated Brian Cowen’s ill-starred 2008-2011 administration.

“It is at the very top of the new Government’s agenda,” said one senior mandarin. Another adds: “Nothing is more important.” So what is the Irish government doing to prepare for a potential Brexit? What can it do?

In the first instance, Ireland wants to make its voice heard in the British debate. That is a delicate operation, skirting the line of foreign involvement in a sovereign decision, but a necessary one, the Government has judged.

The Cabinet, when it meets today, is likely to be briefed on the state of the campaign, and of possible Irish initiatives to assist the British government in its “Remain” campaign.

The Irish position is that it is the British people’s right to decide their future in or out of Europe, but that Ireland has a view on this. In reality, it is a very strong view.

"As friendly neighbours, we respect the fact that this is a decision for the British electorate," the Irish ambassador in London, Dan Mulhall, wrote in a blog this week. "But we do have views on Britain's future in Europe as this is something that will affect Ireland in important ways as a fellow member of the EU and the only country that has a land border with the UK."

Uncertainty

Mulhall has been criss-crossing the UK, meeting Irish-connected groups and pushing Dublin’s message. The London embassy has also been putting together a programme of visits for the coming weeks.

The Taoiseach and his Ministers intend to make a number of trips to the UK to try to help get the Irish and Irish-influenced vote out for the Remain side.

There are an estimated 600,000 Irish born people who are eligible to vote in the UK’s referendum, and millions more with Irish connections whose vote may be influenced by what Irish leaders and Irish organisations have to say.

There are also over 300,000 British or former British-resident people in Ireland who are eligible to vote by post. And there are 1.2 million voters in Northern Ireland.

The British electorate is almost 45 million strong and 30-odd million habitually vote. So the Irish/Irish-influenced vote is a small minority. In such a tight race, however, it could be significant.

But if the Irish Government is working for the best outcome, it is also preparing for the worst. Since Cameron announced that he would hold a referendum if re-elected, officials in Dublin have been trying to scope out what a Brexit would mean for Ireland, and how they can plan to mitigate its worst effects.

Efforts to formulate a contingency plan for a British exit suffer from one great uncertainty – nobody knows what sort of an exit it would be.

“A full plan B isn’t possible yet because nobody can really define Brexit – I mean, what sort of a Brexit would it be?” says one senior source.

Under European treaties, if a member state wants to leave the EU, it opens a two-year negotiation with the rest of the member states. The outcome of this governs its relations with the EU after its exit.

But the process of disentangling from the union would be a complex one. In addition, Britain would have to conclude a trade agreement with the rest of the EU, and with much of the world.

“It would,” one senior British figure told an Irish official recently, “be the most complex thing ever attempted by a British government in peace time.”

Several sources say that two years is an optimistic timetable for such an undertaking.

“There is an argument in Britain that you just repeal the EU treaties, and you get on with it,” says an Irish official in frequent contact with the British. “But that seems unlikely. I think you’d have a typically British approach rather than a Ukip approach.”

Implications

Ireland would be involved in negotiating Britain’s exit with the rest of the EU. But it would also have pressing needs itself to reorder its relationship with Britain outside the EU.

A group of senior officials from across the civil service has been brought together in the Department of the Taoiseach, under assistant secretary Dermot Curran, to try to work out the nuts and bolts of that.

According to people familiar with the group’s work, it has assembled senior officials from the Government departments most directly and obviously affected by Brexit issues – foreign affairs, finance, jobs, agriculture, social welfare, energy and justice – and asked them to report on what issues a British withdrawal might throw up in their areas.

A separate advisory group involving representatives from organisations such as Ibec, the trades unions, Chambers Ireland, the British-Irish Chamber of Commerce, the Institute of European Affairs and the Royal Irish Academy has also been meeting to share research and analysis on the implications for Ireland. The group is meeting this week.

In addition, a group of senior officials led by Rory Montgomery, second secretary general in the Taoiseach's department with responsibility for EU affairs, is advising the Cabinet European affairs committee.

Unpicking and reassembling Ireland’s political, legal and commercial relationship with Britain will be an enormous task.

Close relationships

It is true that institutional and governmental relationships are closer than they have ever been between the two countries, but goodwill alone does not write, say, a trade agreement, or police a border that would be the frontier between the EU and a non-EU country.

One official gives another example of the type of thing that will have to be completely re-examined. In the area of energy, EU countries are required to be able show the Commission that they can withstand their worst winter on record without their major energy asset.

Because Ireland has access to the UK grid through an interconnector, it skates through these tests. But what if Britain leaves? Issues like this will pop up all over Government, he says. And this process would have to happen at the same time that Britain is redefining and recasting its relationship with the EU and the rest of the world.

Britain is an open, globalised nation that trades with the whole world. But the trade agreements that govern most of those relationships are between the EU and third countries, not between Britain and third countries.

Trade agreements are notoriously complex, often taking several years to negotiate, and President Obama recently warned Britain that it would “go to the back of the queue” to negotiate a new US-UK trade agreement if Britain left the EU.

At the same time, Britain would be trying to renegotiate its relationship with the EU, and the terms of its own exit.

Officials say Ireland would be part of the EU negotiations with Britain, but would also have to enter into bilateral negotiations with the British over the border, the common travel area, the North and a host of other issues that link the two countries politically and commercially.

“The idea that we can just cut a side deal with the Brits is naive at best,” said one official.

One of the ways that Irish officials are trying to plan for the contingency they all hope to avoid is to ask themselves questions like: What will be clogging up Liveline the day after a Brexit vote?

“So what about people living here who get UK pensions? Will they still be paid?” wonders another official.

That’s the sort of thing mandarins are now trying to anticipate. Solving these problems if they do arise will be a trickier matter again.