Fáilte Ireland has said The Gathering tourism initiative is already delivering benefits to the Irish economy.
It says the tourist sector has seen its best St Patrick's Day in years, with many hotels in Dublin reporting no vacancies between now and Friday.
The country's biggest parade saw hundreds of thousands of spectators descend on the streets of Dublin.
About 8,000 foreign visitors led the procession in the People’s Parade, which was a Gathering event.
Around 120,000 people passed through the arrivals gate at Dublin airport in the lead-up to this year’s festival, and organisers expect them to contribute towards a €50m financial windfall for the economy.
Fáilte Ireland said it had seen particularly strong levels of visitors from the United States.
Head of communications Alex Connolly said it was clear The Gathering was playing its part.
"The Gathering has been out there for the last number of months as a big hook for visitors to come over. We would be working very hard to get more and more people in this year for The Gathering, and that's what's happened," he said.
"This is the official start of the tourism season."