MICHAEL O'Leary has claimed it was "ridiculous" that hundreds of flights were cancelled because of the latest volcanic ash disruptions.
The Ryanair boss was speaking as Irish airports opened "until further notice" as the threat from the Icelandic plume receded.
He said the system used to measure the risk from the cloud is faulty and insisted the US model should be adopted.
He added that the latest closures removed the last "shred of credibility" of the computer images created by VACC Volcano Concentration Charts.
"Airlines have known for some weeks now that there has been little evidence of any volcanic ash in the atmosphere over Ireland, the UK or continental Europe," he said.
"It is frankly ridiculous that the flight plans of millions of air passengers across Europe are being disrupted on a daily basis by an outdated and imaginary computer-generated VACC model."
Mr O'Leary said "thousands of flights by European airlines had repeatedly confirmed no presence of volcanic ash", reducing "airline confidence in the VACC charts to zero".
He insisted: "It is time to do away with this model now and replace it with the US model which is a practical, safe and sensible response to passenger safety."
The first disruption from the cloud, which lasted five days in April, cost airlines €1.5bn.
Irish airports will remain open until further notice after the latest closures saw 1,000 flights disrupted across Europe.
The Irish Aviation Authority (IAA) said the ash cloud is now moving east and no further restrictions are planned for at least another day when the situation will be reviewed.
Revised safety zones were introduced at midday today, a move which will reduce flight restrictions.
The IAA said "extensive discussions" with airline and engine manufacturers, international regulatory authorities have been ongoing".
In joint agreement with the UK Civil Aviation Authority, the IAA is satisfied the revised zones will reduce the restrictions.