The aviation authority said it was "monitoring the path of the ash cloud", which was around 3,400km long and 2,200km wide.
Ireland's airports have seen several shutdowns in recent days. On Thursday airports were closed before being re-opened just three hours later.
Restrictions on Scottish airspace were also lifted late on Sunday. Flights had been halted in some northern areas, with the exception of the island of Barra in the Outer Hebrides.
Italy and Germany, too, resumed flights, although restrictions remained in parts of Portugal, Spain and Austria.
The closures were expected to last at least until Monday morning.
Portuguese airports cancelled 223 flights, including 119 at Porto and 71 at Lisbon.
French aviation authority said around 30 flights from Paris to southern Europe were also cancelled, although French airspace remains open.
Weather forecasts said that the ash would gradually dissipate as it spreads to southern parts of Germany, the Czech Republic and Austria by Sunday night.
With volcanic eruptions weakening, the plume in the mid-Atlantic was also slowly clearing.
Airspace across Europe was almost completely shut down for around a week last month after Eyjafjallajokull erupted and threw up an ash cloud over most of the continent.
But the authorities reopened airspace and airports after emergency talks between European governments, airlines and regulators.
The international airline industry body, Iata, said last month's shutdown had cost airlines about $1.7bn and called on governments to pick up at least part of the cost.
Eurocontrol said more than 100,000 flights to, from and within Europe had been cancelled between April 15 and 21, preventing an estimated 10 million passengers from travelling.