Plans by aviation chiefs to reopen Irish airspace on Tuesday were thrown into doubt after eruptions at an Icelandic volcano intensified.
The Irish Aviation Authority (IAA) said it hoped to relax an airspace shutdown at 5am but experts warned a new ash cloud was spreading south and east.
European transport ministers devised a plan to reopen skies based on ash levels over three zones across Europe.
But the IAA warned European air traffic control chiefs will decide overnight the size of a no-fly zone, a second buffer zone where ash is recorded and a third area free from restrictions.
"The recommendation of having a no-fly zone around the volcano and a buffer zone, they will determine the size of that," a spokesman said.
"Maybe it's going to be much larger than we thought earlier. If the eruptions get to be a serious level again that no-fly zone will be larger and may extend over Irish airspace."
The IAA said the situation would be monitored constantly with weather patterns and eruptions the key determining factors.
It is feared weary travellers will now have to wait until the end of the week.
Earlier, Aer Lingus said it planned to operate scheduled transatlantic flights as normal on Tuesday as well as flights to Spain, Portugal and Italy up until 1pm.
It had also hoped the majority of European routes would return to normal but those plans were thrown into doubt and the IAA advised travellers to contact individual airlines for the most up-to-date schedules. Ryanair stuck to cancelling all flights until Wednesday.