Passengers passed through Dublin Airport's Terminal 2 today as the new €600m project was officially opened.
Aer Lingus operated the inaugural flight from Manchester as Taoiseach Brian Cowen formally unveiled the new three-storey facility.
Mr Cowen said viewing the multi-million euro project in the context of the economic crisis was short-sighted.
"T2 was designed and built not just for this year or the next, but for many decades into the future," Mr Cowen said.
"To view it through the prism of the current downturn would be short-sighted in the extreme.
"By investing prudently in improved facilities, we are laying the foundations for future growth and prosperity; not just for Dublin Airport, but also for the wider Irish economy."
Construction on the new building began in October 2007 and, at its peak, employed up to 2,600 workers on site, with 1,000 new jobs created with the building's opening.
Dublin Airport Authority (DAA) chairman David Dilger said the project would improve the travel experience for passengers.
He said: "The new terminal is the centrepiece of a five-year investment programme to expand, improve and upgrade Dublin Airport.
"Our passengers told us that they wanted better facilities and we have delivered them."
T2 will be home to Aer Lingus, American Airlines, Continental Airlines, Delta Air Lines, Etihad Airways and US Airways. It will open on a phased basis.
Aer Lingus will have transferred all its services to the new facility by January. It is asking all passengers to come to Terminal 1 and staff will advise where to go.
Etihad will start operating its services from Tuesday.
T2 will also house a new US pre-clearance facility allowing passengers on US-bound flights to clear all customs, immigration, agriculture and security checks before leaving Ireland.
US carriers will transfer when the pre-clearance facility opens in the New Year.
Passengers using pre-clearance at Dublin will be treated as if they are a domestic US passenger and will face no further checks upon entry to the US.
Online travel agency GoHop.ie said people were confused over the new travel arrangements.
Founder Stephen McKenna said: "It is not altogether convenient that passengers have to check into T1 and then travel out of T2, or vice versa for those visiting the country.
"The situation in relation to US customs and immigration needs to be clarified further and implemented as soon as possible before people can enjoy the real benefits of this new terminal."
Ryanair chief Michael O'Leary, a long-standing critic of the project, marked the occasion by dressing like an undertaker with a hearse, a coffin and carrying a wreath with the words Irish Tourism RIP.
T2 also contains almost 40 new shops, food and drink outlets.