Ryanair - whose pilots in Sweden, Belgium and the Netherlands have already voted for industrial action - did not respond to media for immediate comment. The strikes will coincide with stoppages by colleagues in Sweden and Belgium who announced their plans to strike on Wednesday.
Spanish pilots union Sepla has announced it is suing the Irish airline in Spain's High Court over its pilot contracts.
Irish has told over 300 pilots and cabin crew that they may lose their jobs as it cuts its Dublin-based fleet by 20 per cent for the winter season after recent strikes hurt bookings and airfares in its home market.
Ryanair has received a notice from Forsa about a fifth strike to be held on August 10, the company said in a statement.
Ryanair cancelled 24 of around 2,300 daily flights after a second one-day strike by Irish pilots.
Low-priced carrier Ryanair managed to maintain its load factor and increase traffic in July despite having to cancel hundreds of flights over strike action in multiple countries. Sepla represents around 500 of the 800 Ryanair pilots in Spain.
Twenty-five per cent of Ryanair's crew are striking today, marking its fourth strike so far.
Ryanair previously invited Fórsa to talks this coming Tuesday, August 7, in a bid to prevent strike action at Dublin Airport's Terminal 1.
This is the fifth strike by pilots since July 12.
It said that the pilots continue to seek a "fair and transparent method" to govern base transfers and related matters which they say is common practice in the industry.
"This fifth strike notice by Forsa is irresponsible, unwarranted and is disrupting customers and (a way of damaging) Ryanair's business", Ryanair's chief marketing officer Kenny Jacobs said in a statement.
Some 20 Ryanair flights scheduled to fly between Ireland and the United Kingdom next Friday have been cancelled ahead of the day's strike, affecting around 3,500 passengers.