All the world from the Sydney Opera House to the New York Empire State Building turned Irish for the day, as revellers marked St Patrick's Day with bagpipes, dancing, emerald lights and green body paint in a flurry of celebration.

As many as a quarter of a million marchers headed up Manhattan's Fifth Avenue, just as crowds gathered along parade routes in sunny Dublin to mark the emerald-hued holiday.

It was expected to be a mix of lighthearted cheer and serious politics at the White House, where President Barack Obama met Irish Prime Minister Brian Cowen.

Obama noted that 36 million Americans claimed Irish ancestry, adding: "I'm sure more do on St Patrick's Day.

"And it's a testament I think to how close our two countries are that America has been shaped culturally, politically, economically by the incredible contributions of Irish Americans," Obama said.

As part of a marketing deal by Ireland's tourism agency, major world landmarks - including the Sydney Opera House, the London Eye, Toronto's CN Tower and the Empire State Building - were bathed in green lights.

St Patrick's Day is Ireland's first major tourist event of the year, packing hotels and pubs with visitors seeking an all-night party.

The 249th St Patrick's Day extravaganza will be the last of New York City's world-famous parades to take place before new restrictions go into effect on April 1 requiring all parades to be shorter to save money.

Tourist John Rupy considered this news along the parade route, where he arrived dressed in a kilt with his skin painted green.

"That's not good," he said, "because the whole world comes to this."

Parade participants included the "Fighting 69th," a New York National Guard unit whose history stretches to the US Civil War when immigrants made up the so-called "Irish Brigade" of the Union Army.

The day is named after St. Patrick, who introduced Christianity to Ireland about 1,500 years ago and became the country's patron saint.