Ireland's longest tunnel is now in Co Mayo and is 4.9kms long.
The Corrib Gas tunnel took the top spot from Dublin's Port Tunnel, as it is 300 metres longer.
It runs under Sruwaddacon Bay and will house an onshore section of the Corrib gas pipeline.
More than 1,000 people were part of the construction which began in January of last year. Half of those employed were locals from the Mayo area.
Managing director of Shell E&P Ireland Michael Crothers said that despite past resistance to the project, the tunnel is an example of engineering excellence in Ireland.
"There isn't much protest left any longer," he said. "We acknowledge mistakes we made many years ago, but there has been a huge amount of leanring and we've really tred to balance the economic with the social and environmental (factors).
"The feedback we get from the community is really positive. We don't take that for granted."