A man dressed as St. Patrick poses for pictures as the Sydney Opera House is lit with green lights during St. Patrick's Day celebrations in Australia. (Daniel Munoz/Reuters)

Much of the world is turning green Wednesday for St. Patrick's Day, the annual celebration all things Irish.

More than half a million people lined the three-kilometre route of the flagship Dublin parade, which is exploring the theme "The Extraordinary World."

It's a nod to Ireland's increasing multiculturalism — as well as the past two centuries' global spread of the Irish.

This year Ireland is also promoting itself especially hard as a tourist destination as the country faces its worst recession since the Great Depression, with double-digit unemployment and net emigration for the first time in 15 years.

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. Ireland's weeklong festival gets bigger each year, with more than 100 parades Wednesday in cities, towns and villages across the island of six million.

The Tourism Ireland agency wangled a deal for major world landmarks — including the Sydney Opera House, London Eye, Toronto's CN Tower and New York's Empire State Building — to be bathed in green floodlights as part of a marketing push on four continents.

Irish PM visits Washington

U.S. President Barack Obama finishes his meeting with Ireland's Prime Minister Brian Cowen in the Oval Office of the White House on St. Patrick's Day. (Charles Dharapak/Associated Press)

Virtually the entire Irish government left the country this week to meet with foreign leaders and corporate kingpins in 23 countries in hopes of rekindling the investment wave that fuelled Ireland's Celtic Tiger boom of 1994-2007.

Prime Minister Brian Cowen met U.S. President Barack Obama at the White House, continuing Ireland's unique tradition of annual access to perhaps the world's most powerful man.

President Mary McAleese, Ireland's ceremonial head of state, who stayed at home to preside over the Dublin parade and her own St. Patrick's garden party, said the Irish had powerful allies in politics and business backed by 70 million people of Irish descent, half of them Americans.

"We are lucky to have such a large global family. It has proved itself to be a very precious and important resource in every generation," she said.

The Catholic Church, the once-dominant Irish faith, sought to remind revellers of the true story of Patrick: a Briton enslaved in his youth in Ireland who returned to spread Christianity throughout the pagan land in the fifth century.

Bishop Seamus Hegarty's St. Patrick's Day message called for prayers for immigrants — both the Irish seeking jobs overseas and the Emerald Isle's own tens of thousands of newcomers from Eastern Europe, Africa and Asia who often work the longest hours for least pay.

"Mindful that St. Patrick was himself a migrant, we as a people are called to build a society that is truly inclusive, a society that is welcoming and respectful of people of different cultures, languages and traditions," Hegarty said.

"While our primary focus must be to ensure that we prevent another lost generation, we must also ensure that for those who decide to emigrate, they are neither abandoned nor forgotten," he said.