Irish airspace was re-opened for the first time in six days last night, but the Irish Aviation Authority (IAA) warned that could change if the ash cloud moved over the country during the night.
The decision was also made on the basis of there being no more reported aggressive activity from the Icelandic volcano.
The promise of relief for tens of thousands of air passengers came in a 9pm statement from the IAA which said it and the UK's Civil Aviation Authority had sanctioned the resumption of the full service on a phased basis in all Irish and UK airspace to Europe, North America and the UK.
However, it is expected that full service by the airlines may take up to three days to recover, the IAA warned.
The authority said that following the agreement of the European transport ministers on Monday, it, and the UK CAA, had agreed on a safe model to allow flight operations to resume in both countries.
Eamonn Brennan, IAA chief, said: "We welcome this agreement, which balances safety priorities with the need of the industry.
"This provides the foundation for a long-term solution to the volcanic ash issue. However, we need to be careful over the next few days as we are concerned that the ash cloud currently positioned between Iceland and Ireland may drift over the country. Ireland is one of the closest European countries to the exclusion zone," said Mr Brennan.
ADD Dublin Airport Authority advised that only passengers with confirmed reservations for flights due to operate today should go to Dublin, Cork or Shannon airports.
It warned that there would be limited flight operationsat the airports as "airlines try to recover and resposition aircraft" in preparation for a full return to their flight schedule.
Passengers were advised to consult with their airline's websites before travelling to the airports.
Subject to Ireland not being in the high-risk exclusion zone, and the continued dispersal of the volcanic ash cloud and no further significant eruptions, restrictions in Irish airspace ended last night at 9pm.
Aircraft engine manufacturers have been consulted and detailed technical guidance has been developed in conjunction with CAA and IAA, the Authority said.
"This has been issued to airlines and maintenance organisations. This includes a risk-management regime and additional maintenance. Each airline will have to comply with guidance material that will include additional reporting, maintenance inspections and regular ash ingestion assessments.
A "big change" in weather conditions forecast for Friday and Saturday should see the ash pushed north towards Greenland, allowing aircraft to resume flying and for tens of thousands of stranded air passengers to get home.
Met Eireann said the prevailing wind was expected to blow south-westerly by Friday and this should help to get rid of the ash cloud hanging over the country closing airspace.
At 9pm on Monday, the Irish Aviation Authority announced the easing of flight restrictions. But it was forced to pull the plug on the lift-off plan just four hours later at 1am. This was after a new ash cloud quickly spread south and east towards Ireland and the UK.
A spokesman for the IAA told the Irish Independent yesterday that it had decided to wait for key advisory maps to be sent over from the Volcanic Ash Advisory Centre in London, setting out the precise progress of the ash before issuing a public update.
The maps, showing the cloud intensifying and heading over Ireland, arrived at 12.45am yesterday. Based on the new information a decision was taken by the Irish authority to reverse the earlier decision to ease flight restrictions. and this was announced at 1am.
Met Eireann said that it received information from the Icelandic Meteorological Office on Monday suggesting the ash emissions had slowed down and in fact had virtually ceased for a time.
"But the volcano reactivated again on Monday evening and overnight," it said.
A Met Eireann spokesman said that the unexpected overnight eruption took between six to 10 hours to arrive here, blown by very strong north-westerly winds.
As many as 65,000 passengers who had flights cancelled will be trying to get the first available seats when the crisis eventually eases, it was revealed yesterday.
Aer Lingus said that, while at least 80,000 passengers were due to fly when the flight ban was imposed, just 15,000 passengers had asked for refunds.