His jacket was only moss green but his pint was true Guinness.

US President Barack Obama tilted back a glass of the dark Irish brew on Saturday, observing St. Patrick's Day at a boisterous Irish pub with his ancestral cousin from Moneygall, Ireland, at his side.

At the White House, the main South Lawn Fountain burbled green water. Nearby, workers prepared for a visit on Tuesday by the Irish Prime Minister Enda Kenny.

The first family was putting on its Irish, a bloodline that runs through Obama's veins.

Obama took his motorcade to the Dubliner Restaurant and Pub on a dazzling Saturday afternoon. He wore no Kelly green but his jacket was pierced with a button that read, "VIP GUEST - Tell 'em Danny sent you". The president waded into a crowd - some in leprechaun hats and others in dyed green hair - at the entrance of the tavern near Washington's landmark Union Station.

He wished one reveler, Adam Joseph, a happy 29th birthday.

Reporters were ushered into the pub briefly, long enough to catch the president taking two sips of his beer. The thick foam stuck to his upper lip in a thin mustache. And he held Dubliner owner Danny Coleman's 21-month-old grandson, Danny, in his arms.

"Hey, Danny, who's this?" Obama asked, holding out his VIP GUEST button, which bore the child's image.

One of Obama's great-great-great grandfathers on his Kansas mother's side was Falmouth Kearney, a shoemaker who emigrated from Ireland to the United States in 1850. Last year, Obama visited his ancestral home of Moneygall, a small hamlet in Ireland, and was a hit when he drank a Guinness at the local pub.

On Saturday, the owner of that pub, Ollie Hayes, and Henry Healy, an eighth cousin to Obama and the closest relative still living in Moneygall, joined him barside at the Dubliner as his guests.

Dubliner owner Danny Coleman said the president finished his beer, and predicted that, at least on this day, Obama could count on the support of the millions of US citizens who claim Irish ancestry.

On Tuesday, Obama and Vice-President Joe Biden will meet the Irish Prime Minister Enda Kenny and attend a St. Patrick's Day lunch at the Capitol.