An irregular software issue was to blame for most of Ireland's airspace being closed earlier this week.
A backup system was deployed on Tuesday night when air traffic control in Shannon and Cork failed.
Following an investigation, the Irish Aviation Authority says the issue was not related to radar, but was a software problem.
Operations have now been transferred back to the primary system which has been fully restored.
The failure at Shannon Air Traffic Control (ATC) centre on Tuesday night forced the closure of Irish airspace to overflights, the rerouting and grounding of dozens of flights, and the deployment of an Emergency Air Situation Display System (EASDS) in an effort to maintain safety in the skies.
The IAA held the investigation to find the root cause of the issue, and said its systems were complex.
Yesterday, its director of commercial technology, Philip Hughes described the breakdown as “highly rare”.
He said it had happened at “a relatively quiet time” — before the overnight transatlantic overflight traffic kicked in.
The IAA’s Area Control Centre in Shannon handles over 90% of all air traffic on the North Atlantic. This equates to approximately 1,400-1,500 aircraft every 24 hours during the busy summer months, according to its website.
In 2017, Shannon Air Traffic Control safely handled over 343,000 flights.
Digital Desk and Catherine Shanahan