On St Patrick’s Day the whole world is, of course, Irish; but for the first time in that long tradition of Celtic mischief an Irish Prime Minister managed to be US President for a brief, shining moment.

It happened at the White House, where President Obama’s welcome to Brian Cowen extended to turning the ornamental fountain a gushing green, serving green champagne cocktails — and almost turning the Irish Prime Minister’s face red with embarrassment over a mix-up with the speeches.

The gaffe was a result of President Obama’s fondness for the Autocue and of the large number of guests at his St Patrick’s Day dinner. With 300 people invited — the biggest White House party since his inauguration — the event had to be split between two rooms. President Obama, Vice-President Biden and their guest of honour, Mr Cowen, all had to deliver their speeches twice: first in the East Room and then in the State Room.

The first run went smoothly. Mr Biden got a roar of approval when he noted that the British withdrawal from Boston during the War of Independence had taken place on March 17. But by the time the two leaders had trooped across the White House for their second speaking engagement of the evening, some leprechauns had got at the Autocue and loaded the speeches in the wrong order.

So it was that Mr Cowen — “Biffo” to his friends and enemies back home, an acronym of dubious provenance — found himself briefly transformed into the holder of the Seal of Office.

“We begin today by welcoming a strong friend of the United States,” he intoned grandly, wondering to himself by what right he was doing that.

Luckily, it rapidly dawned on the Taoiseach that one’s words can often go awry on Paddy’s Day for reasons other than an excessive intake of stout, one of the festival’s cardinal rituals.

“This is your speech!” he told President Obama, gesturing at the flickering screen before him.

The State Room dissolved into laughter as Mr Cowen turned an awkward moment to his advantage by adding: “Why don’t these things work for me? Who said these things were idiot-proof!”

President Obama graciously stepped in and resumed his elected role from Mr Cowen, but not before a quick improvisation — or maybe he was now reading the Taoiseach’s speech from the Autocue: “First of all I’d like to thank President Obama . . . ” The roomful of guests collapsed into mirth once more.

Call it the Luck of the Irish but the slip-up certainly made this White House audience with President Obama — only the third to be granted to a government leader — one that will not soon be forgotten.

Mr Cowen even taught his host a few words of Irish: “Is féidir linn”, which means “Yes we can”. The phrase may come in handy one day. President Obama revealed that his maternal great-great-great-grandfather came to America from a village in Co Offaly, where Mr Cowen was born and brought up. “We are still speculating on whether we are related,” said the President, to another round of laughter.

He also suggested that the reason that Guinness tasted different in Ireland was that the Irish were keeping all the best casks for themselves and that a trade dispute might consequently erupt.

The President was in full Paddy’s Day mode by now, quipping to the bibulous throng as he tripped from one room of guests to another: “You can stay as long as you like but try to avoid putting any lampshades on your head — there are photographers about. . .”

As the St Patrick’s Day events drew to a close, Mr Cowen — by now practically a country cousin of the President — said: “We’ve done a hard day’s work. Now it’s the time to have the craic, as we’d say at home.” Lest there be any misunderstanding over the differences of meaning between “craic” and US-style “crack” the Taoiseach hastily added: “I hope everyone understands that’s an Irish thing.”

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