The Gathering officially kicks-off with a New Year's Eve festival in Dublin city centre tonight.
It will be the first in a year-long calendar of events organised to celebrate Ireland, its people and its connections, both at home and abroad.
Thousands of people have attended the People's Procession of Light and a fireworks display ahead of a countdown concert in College Green, headlined by Imelda May and Bell X1.
Organisers say journalists from around 70 international broadcast and print media organisations have arrived in Dublin to cover the event.
The festival will be broadcast live from 10pm on RTÉ Radio One, and from 10.30pm on RTÉ One Television and online.
A fireworks display was also held in Dún Laoghaire at 6pm and there will be a second display at midnight.
In Co Cork, the start of The Gathering will be marked with a New Year's Day 21-gun salute on Spike Island.
Meanwhile, Derry also marked the start of its year as the UK city of culture.
Thousands of people attended a spectacular fireworks display along the River Foyle and across the new peace bridge.
The year-long programme of cultural events will include hosting the Turner Prize and All-Ireland Fleadh Ceoil along with visits by the Royal Ballet Company and the London Symphony Orchestra.
Elsewhere, Sydney, Australia, welcomed the New Year in spectacular fashion.
A pair of giant red lips atop the Sydney Harbour bridge counted down the seconds to midnight.
The city exploded in colour as fireworks were launched from seven skyscrapers, with seven tonnes of pyrotechnics fired from seven barges on the harbour.
In Dubai, a spectacular fireworks display illuminated the Burj Khalifa, which is the tallest building in the world standing at 828m high.
In Russia, Moscow's iconic Red Square was filled with spectators as fireworks exploded near the Kremlin to welcome in the new year.
Earlier, about 25 people were reportedly arrested in Moscow for trying to hold an unsanctioned demonstration.
But President Vladimir Putin gave an optimistic New Year's Eve address, making no reference to the anti-government protests that have occurred in his country in the past year.
"We believe that we can change the life around us and become better ourselves, that we can become more heedful, compassionate, gracious," Mr Putin said, according to the ITAR-Tass news agency.
In North Korea, cannons boomed at midnight in Pyongyang as people crowded the streets of the capital to watch a fireworks show over the Taedong River.
After being in mourning a year ago over leader Kim Jong Il's death, North Koreans celebrated the end of a big year that included the rise of new leader Kim Jong Un and the recent launch of a satellite into space.
Many New Year festivities in India's capital New Dehli were cancelled following the death on Saturday of a young rape victim.
People were asked to light candles to express their solidarity with the victim.