Unions representing Ryanair cabin crew in the countries affected say they are taking the action after the airline refused to meet a deadline for a series of demands over employment conditions.
The dispute is over base transfers, promotions, leave and various other issues.
After Friday's strike a third 24-hour walk-out is planned for next Tuesday.
A senior official at the airline said he has high hopes the Irish Airline Pilots Association division of the Forsa union will call off next week's stoppage. But management are believed to be disappointed to have made less progress placating its Irish pilots then it has with British and Italian unions.
Ryanair has over 250 self-employed pilots which allows the airline to operate most of its routes during the strikes.
The majority of pilots flying Ryanair planes are self-employed, enabling the airline to restrict the impact of the strikes.
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Up to 300 flights a day to and from the aforementioned countries - which accounts for about 12 per cent of the airline's 2,400 daily European flights - have been cancelled, affecting 50,000 customers on each of the days.
"It is too early to say that there is any implication to Ryanair's business in Ireland at this stage but everyone should remember aircraft are moveable assets", said Ryanair chief marketing officer Kenny Jacobs.
But it has since struggled to reach agreement on terms with a number of them.
The airline's already cancelled 24 flights between Ireland and the United Kingdom on Friday, ahead of the strike. Anyone who is traveling on a cancelled flight should have been informed via text message or email.
"All customers on these flights have already today received text and email notification of these cancellations and our Customer Service teams are assisting them with refunds, and free transfers to alternative flights on Wed, Thu, Sat or subsequent days".