Donald Trump's election as US president poses a major threat to an already-slowing Irish economy. If he fulfils his promise to slash the US corporate tax rate to 15pc, it will seriously undermine Ireland's attractiveness as a location for foreign direct investment.

In the 19 months since June 2015, when he first announced his intention to seek the Republican Party nomination, the prospect of a Trump presidency has inexorably moved from the realm of the unthinkable to that of the unstoppable.

President-elect Trump's promises to "bring back American jobs", tear up the North American Free Trade Agreement, not ratify the Trans-Pacific Partnership, label China a "currency manipulator" and slash the US corporate tax rate to 15pc - all of which had been widely dismissed as campaign bluster - must now be taken seriously.

If The Donald moves to deliver on these promises then the implications for Ireland Inc could be severe. A global trade war would hurt the world economy. With one of the most open economies on the planet, Ireland would suffer disproportionately from such a trade war.

Part of the problem when seeking to predict the implications of a Trump presidency for Ireland is that he has said so many, often contradictory, things in his quest for the presidency.

"If he cuts personal and corporate taxes and increases infrastructure spending, that will deliver a huge stimulus to the US economy. This will suck in imports and push up the dollar," says Professor John FitzGerald, adjunct economics professor at TCD.

"On the other hand, there is his rhetoric on trade. If President Trump ripped up international trade agreements, then that would be negative for the world economy."

Of most concern for us in this country is President-elect Trump's pledge to cut the US corporate tax rate from its current 35pc, one of the highest in the developed world, to 15pc - within spitting distance of Ireland's 12.5pc rate. Just for good measure he has also promised to scrap tax deferral for the unrepatriated overseas profits of US companies and introduce a special 10pc rate instead. With US companies sitting on an estimated $3 trillion of unrepatriated overseas profits, this could produce a massive payday for the Internal Revenue Service.

The success of President-elect Trump's Republican Party in retaining control of both houses of Congress means that he is uniquely well placed to deliver on his campaign promises. After years of political gridlock in Washington, the new president is widely expected to move quickly on corporate tax reform following his inauguration on January 20. Senate and House majorities mean that a cut in the US corporate tax rate could be quickly enacted.

What will that mean for us here in Ireland? IDA-supported companies directly employ almost 180,000 jobs with almost as many more jobs indirectly reliant on the multinationals. That's almost a quarter of total private sector employment.

The multinationals also contribute the vast bulk of the revenue from corporation tax, with an estimated 80pc of the 2015 total of €6.9bn (more than €5.5bn) coming from this source.

While the new administration may move quickly to cut the US corporate tax, it is likely to stagger the implementation of any reduction. This is because of the prohibitive cost - it has been estimated that bringing the US corporate tax rate down to 15pc would cost somewhere between $4.4 trillion and $9 trillion over the next decade. Even for an economy as large as the United States, that's very serious money.

"He [Trump] has to do something. They will signal it [the cut the US corporate tax rate] over seven to 10 years", says FitzGerald.

However, even if the implementation is phased, the enactment of the cut will surely alter the behaviour of US multinationals. Why locate a plant in Ireland with its 12.5pc corporate tax rate if you can benefit from a 15pc rate by staying at home?

That's not quite how FitzGerald sees it. He points out that if Trump triggers a trade war then US companies will need a European base to circumvent any retaliatory tariffs imposed by the EU. Ireland, with its 12.5pc tax rate, will remain an attractive location for US companies with European operations.

Ironically, Ireland could benefit, at least in the short term, from a trade war, FitzGerald believes.

The repatriation of US companies' overseas profits may also have less of an impact than has been feared.

"It's back to the future. The US did it [introduced an amnesty for repatriated profits] already in 2004. It did not have a large impact on Ireland. It may have slowed down or reduced investment but we saw nothing moving out," says Joe Tynan, head of tax at accountants PwC.

Even if the new administration does get its proposed 15pc corporate tax rate onto the US statute book, many companies will probably adopt a wait-and-see attitude before moving back to the US. They will want to be sure that the reduction is for keeps rather than just for the duration of a single four-year presidential term, says Tynan.

"It is very difficult to get out of the US," he says.

What is likely to happen is that US companies will be less inclined to structure their businesses in such a way as to get profits into Ireland.

This could see some of the "Irish" companies created by the tax inversions of recent years upping sticks.

While most attention in this country has been focused on President-elect Trump's tax and trade policies, Tynan believes that the campaign pledges he has made on immigration could also work to Ireland's advantage. An estimated 50pc of US high-tech startups are founded by immigrants. With the incoming president having described Mexicans as "rapists" and promised to ban Muslim immigration, how welcome will these immigrant founders feel in Trump's brave new world?

Will we be able to entice some of them to locate their startups in Ireland?

A 15pc US corporate tax rate would certainly make it harder for Ireland to compete for investment by American companies. On the other hand, we would certainly acquire a powerful ally in our battle with the EU in the Apple tax case.

Trump threatened to invoke Section 891 of the US tax code, which allows the IRS to double the tax rate paid by companies and citizens of countries judged to be discriminating against US companies. That's a pretty heavy stick to beat the EU Commission over the head with.

Such a silver lining, while welcome, doesn't take away from the fact that Trump's sums don't add up. Massive tax cuts and more spending on infrastructure can only be funded by higher borrowing.

We may all end up paying the price of Trumponomics in the years ahead.

Sunday Indo Business