Low-cost transatlantic flights will link Belfast to the US East Coast, it has emerged.
Norwegian Air says it will also serve Dublin, Shannon and Cork from July.
The carrier is promising fares from 69 euro or £69 to secondary airports in the New York and Boston areas.
With extra charges for meals and hold baggage, its chief executive has been quoted as saying tickets should on average cost about $300 to $350 for a return.
Five weekly flights will leave Belfast International while 19 will depart from the Republic.
Those between Dublin and New York will operate daily while services from Belfast, Cork and Shannon will be less frequent.
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The US airports served are Stewart International, which is less than 90 minutes from New York City, and TF Green, which is 90 minutes from Boston.
The smaller airports carry significantly lower landing charges than traditional hubs such as JFK, which Norwegian said enabled it to offer more affordable fares.
Routes will be operated on new Boeing 737 MAX aircraft with a single economy class.
The airline said it was state-of-the-art and offered longer range and more seats.
Norwegian CEO Bjorn Kjos said: "With the first ever transatlantic flights from Cork and new services from Dublin and Shannon, we are shaking up transatlantic travel with ground-breaking fares and never before seen routes.
"The cost of transatlantic travel has been too high for too long so by connecting Irish cities with smaller US airports, we can offer some truly affordable fares, allowing as many people as possible to fly.
"We are delighted to announce our first ever flights from Belfast and to ensure the city maintains its crucial transatlantic links."
Dublin Airport managing director Vincent Harrison said the volume of transatlantic traffic has tripled since last year, with 19 new services added since 2011.
Belfast International has been working to find a replacement to United Airlines, who ended its New York route in January.
United had claimed the Belfast-to-Newark service was not profitable enough.
Belfast International director Graham Keddie said: "This is a great day for the airport, Northern Ireland, indeed, the entire northern half of the island of Ireland."
Norwegian originally intended launching services from Cork and Shannon last year, but a delay in getting approval from the US authorities prevented this.
Shannon Airport managing director Andrew Murphy said: "It reaffirms yet again Shannon Airport's status and contribution as the premier gateway between this side of the island and the US."
Tourism Ireland chief executive Niall Gibbons said: "As an island, the importance of convenient, direct, non-stop flights cannot be overstated - they are absolutely critical to achieving growth in inbound tourism."