Tourism Ireland has said it is impossible to put a price on the publicity that Ireland receives around the world on St Patrick's Day.
More than 190 landmarks around the world are taking part in the Global Greening campaign to celebrate our national holiday.
Iconic buildings and monuments going green today include the Great Wall of China, the London Eye and the 7 World Trade Centre at Ground Zero in New York
CEO of Tourism Ireland Niall Gibbons said the worldwide campaign is worth millions in advertising and publicity.
"If we were to buy the publicity we're getting today, it would cost us at least €20m," he said.
"For example, NBC last year on their nightly news, coat-to-coast across America, ran a 30-secopnd slot (on the Greening). If we were to buy that, it would costs us $2.2m.
"All national newspapers across the UK ran the pictures last year, and we expect the same to happen this year."
Sydney Opera House was the first glocal landmark to go green, in 2010. This year, about 180 landmark buildings and iconic sites across the world will turn a shade of green for our national day.
More than 100,000 tourists have come to Ireland for the festivities, with half a million expected to line the streets of Dublin for the largest parade in the country. It starts at 12 noon, from Parnell Square North to St Patrick’s Cathedral.
The Grand Marshall of the Dublin parade is disability rights activist Joanne O'Riordain.
A larger than normal garda presence will be seen in the capital to clamp down on anti-social behaviour and to help deal with the crowds.
The good news for those planning to go to any of the St Patrick's Day parades around the country is that it should stay dry.
Met Éireann's Jean Byrne said it would be "pretty pleasant, certainly dry everywhere with most areas enjoying good, sunny spells".
The Cork City parade starts from South Mall at 1pm; Limerick's at 12 noon from Pery Square and Galway's at 11.30am from Dominick Street. The Waterford parade starts at 1pm on The Quays.
Meanwhile, an historic St Patrick's Day Parade will take place in New York City today.
The event will include Irish LGBT groups marching behind their own banners for the first time.
Minister for Foreign Affairs and Trade Charlie Flanagan TD will be there to represent the Irish Government
A total of 415 Irish soldiers will be spending St Patrick's Day on peace missions in 15 countries.
The peace support missions include Mali, Kosovo, Lebanon and The Democratic Republic of Congo.
The troops will celebrate St Patrick's Day with a mix of military and cultural traditions.