"It is probably one of the biggest global days around the world, when the Irish tribe showcases all things Irish," Tourism Ireland chief Niall Gibbons told AFP.
"If you're the president of any corporation and you were offered a chance where cities and world leaders stop and hold civic receptions and celebrate your product -- you just wouldn't be able to put a value on it. It's priceless."
Political and tourism chiefs see the weekend-long cultural festival as a chance to repair some of the damage the eurozone crisis has done to Ireland's international reputation.
From Egypt's pyramids to the Sydney Opera House, some 70 global landmarks will be bathed in green light in honour of Ireland's national day -- while in Dublin itself, more than half a million people were expected to line the streets for Sunday's colourful annual parade.
With marching bands, dancing troupes and, of course, copious quantities of Guinness flowing, officials hope the crowds will spend heavily in Dublin's pubs, shops, hotels and restaurants as they "paint the city green".
Dublin's Chamber of Commerce hopes the weekend will give city businesses a 50-million-euro ($65 million) boost.
Guinness is one company set to do well out of St Patrick's Day -- the Dublin-based brewer expects 13 million glasses of its stout, affectionately known as "the black stuff", to be drunk across the world on Sunday alone.
Famed for its double-digit growth in the 1990s, Ireland's once-proud "Celtic Tiger" economy crashed in the late 2000s when a property bubble burst.
The government was forced to seek an 85-billion-euro bailout from the European Union and International Monetary Fund in 2010.
While Dublin appears to be pulling itself away from the brink of financial collapse, growth remains feeble -- with the economy expanding just 0.2 percent in the last quarter.
"We've had a pretty terrible external reputation over the course of the last five years since the economic crash," Gary Murphy, professor of government at Dublin City University, told AFP.
"If Ireland prides itself as a good country to do business and visit, it's important we sell ourselves as such to the world."
Irish Prime Minister Enda Kenny and 19 members of his cabinet have travelled to 21 countries across the globe this weekend, selling "brand Ireland" in a series of trade, investment and tourism-focused meetings.
In New York, where an estimated two million people turned the city into the Big Green Apple with a massive parade on Saturday, Kenny urged the estimated 40 million Americans with Irish ancestry to return home for a visit.
"You'll be spoiled for choice," he told a breakfast with New York Mayor Michael Bloomberg.
His invitation is part of a government-supported initiative called "The Gathering" aimed at encouraging the huge Irish diaspora, which is 60-million strong by some estimates, to come and connect with their roots -- and spend some cash.
For the first time ever, visitors from abroad were invited to march in Sunday's parade through Dublin, and almost 6,000 people from Mexico to Malaysia registered to take part.
Ireland has introduced a range of other measures to boost tourism, including cutting taxes on the hospitality sector and visa waivers for some countries outside the EU.
"St Patrick's Day is an icon of Irishness the world over and from a business point of view, it's hugely important to the tourism sector in Dublin and essentially kicks off the Irish tourism season," said Gina Quin, president of Dublin Chamber of Commerce.
"We expect 800,000 people to be involved in the festivities in the city with 120,000 of those fresh visitors coming from overseas," she added.