President Barack Obama charmed the thousands of fans outside Dublin’s College Green by uttering cupla focal.
“Ta athas orm a bheith in Eireann” (I’m happy to be in Ireland),” he said to huge cheers.
He added he wished somebody had traced his Irish roots when he was a politician in Chicago, which he dubbed the Irish capital of the mid-west.
He revealed he had craved a slot in that city’s St Patrick's Day parade, only to be given the final place ahead of the street cleaners.
"I bet those parade organisers are watching TV today and feeling kind of bad because this is a parade right here," he said.
The President also paid tribute to his eighth cousin Henry Healy having visited his ancestral homeland of Moneygall, Co Offaly earlier and entertained other distant relatives as they enjoyed a drink in the local pub.
The President praised the work involved in achieving peace in Northern Ireland.
"America will stand by you always in your pursuit of peace," he said.
"Ireland, you need to understand that you have already passed the world's highest hopes."
He added: "A dream has turned to a reality because of the work of this nation."
The President met Northern Ireland First Minister Peter Robinson and Deputy First Minister Martin McGuinness before making the speech.
In a resounding address, the President said: "This little country that inspired the biggest things - your best days are still ahead of you."
Meanwhile, President Obama said the US and Ireland has a centuries-old relationship and that this country’s best days are ahead of us.
"And that's why I can come here today, as American President, and reaffirm these bonds of affection," he said.
After a 24-minute speech, the President went down from the stage and shook hands and chatted with well-wishers lucky enough to be at the front of the massive crowds.