Some 407,100 more visitors came to Ireland in the first half of this year compared to 2014 - an increase of 11.7%.
The number of North American visits increased by 14.7% to 659,200 visits in the period, meaning that one in 10 US visitors to Europe now visit Ireland, according to CSO figures.
The number of visits from Mainland Europe grew by 13% to 1, 376,900 visits, with Great Britain up by 9.4% (1.612 million visits).
Visits from the rest of the world, mostly long-haul and developing markets, were up by 12.1%, totalling 228,000 visits.
Some 3,876,200 visits were recorded in total.
Niall Gibbons, CEO of Tourism Ireland, said: "Today’s figures indicate that this was the best ever January to June period for overseas visitors to Ireland.
"Today’s figures very much reflect the sentiment expressed by our tourism industry partners overseas, including tour operators and carriers, as well as tourism businesses here at home.
"Promoting the Wild Atlantic Way and the Causeway Coastal Route continues to be a major focus for Tourism Ireland around the world in 2015; we are also showcasing the new Dublin Discovery Trails and paving the way for Ireland’s Ancient East and the Gobbins Cliff Path. 2015 marks 150 years since the birth of Nobel laureate, WB Yeats, and Tourism Ireland is promoting Yeats2015 – particularly to our ‘culturally curious’ audience.
"This year has also been designated ID2015 (the year of Irish design), a showcase of the best of Irish design."
Minister for Transport, Tourism & Sport, Paschal Donohoe said: ‘The latest publication shows that we are doing well from all of our main target markets including Mainland Europe, Great Britain and North America and confirm the signals we have been getting from the trade.
"The newly formed Tourism Leadership Group, which I will chair, will meet in the coming weeks to start work on the development of a Tourism Action Plan to specify actions to achieve the ambitious growth targets for Irish tourism set out in the Government’s Tourism Policy, 'People, Place and Policy - Growing Tourism to 2025'.’’