An empty Ryanair customer services desk is seen at Dublin airport in Dublin, Ireland September 27, 2017.
The Irish Airline Pilots' Association (IALPA) said that 99 per cent of over 100 directly-employed pilots who were balloted backed industrial action in the vote.
The union said Ryanair was not taking their demands over working and pay conditions and how annual leave is dealt with.
A spokesman for Ryanair was not immediately available for comment.
Cabin crew in Spain, Portugal, Italy and Belgium are planning the strikes and will announce dates for the action on Thursday once authorities in Italy give their approval, Antonio Escobar, an official with Spanish union SITCPLA said, adding that other countries may join the action.
The strike is planned for Thursday July 12, for 24 hours.
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It's reported that the pilots, who are directly-employed by the airline, voted for strike action following a dispute over management's approach to transferring pilots between its European and African bases.
In a letter to national secretary Angela Kirk, he said the airline had offered talks on pilot demands for a "seniority" system for allocating holidays, promotions and base transfers.
Speaking after a summit in Dublin, representatives of cabin and ground crew from across Europe said there had been little progress since O'Leary reversed a long-held policy of refusing to recognise trade unions more than six months ago.
The airline's already had to cancel flights because of broader air traffic control strikes and staff shortages that have affected the industry.
This compared with just 41 cancellations in June previous year.
It has been six months since the last threat of strike action within the company.
Ryanair claims it has invited Forsa and IALPA to meet "on 18 separate occasions" to discuss why pilots find the agreement unfavourable, with "no reply".