Irish-based Ryanair pilots who are members of the Irish Air Line Pilots' Association have voted overwhelmingly to back industrial action.
The union said it would write to management early next week to outline plans for action unless the airline agrees to union pay proposals by Monday.
It says the airline is in a healthy financial position and can fairly reward its pilots.
The IALPA members voted by 94% in favour of industrial action and the union will have to serve notice of at least seven days to the airline before it can take place.
IALPA represents 180 of Ryanair's Irish based pilots.
When it announced the ballot, IALPA, which is a branch of Fórsa, said the decision reflected the frustration and disappointment of pilots with the lack of progress in ongoing pay talks with the company.
Fórsa assistant general secretary Ian McDonnell said industrial action could be avoided if management at the airline engaged professionally and constructively in talks.
He accused the company of stalling tactics in negotiations.
"Ryanair's directly-employed Irish-based pilots are simply seeking pay levels that are common and competitive in the commercial airline sector, from a company that made a more-than-healthy profit of €1 billion last year.
"They feel they have been forced into contemplating potentially disruptive industrial action by a company that seems either unwilling or unable to negotiate in a professional and constructive manner. At this stage, only a substantive counter-proposal, which properly addresses all areas of our claim, will be enough to prevent us serving notice of industrial action next week," he said.
Responding to the vote, Ryanair said it was "disappointed" that the union was "threatening to disrupt customers' travel plans during August".
It claimed the threatened strike was supported by a minority of Ryanair pilots and was ill-timed given the looming Brexit deadline.
It also accused the union of failing to present its pay proposals to Kieran Mulvey, who is mediating talks between the two sides.
"Fórsa are still unable to explain what pay increase they are seeking on top of the 20% increase already agreed," Ryanair said.
"At a time when Ryanair pilots resignations have dwindled to zero because Ryanair pilots are better paid than 737 competitors in Norwegian and Jet2."
Ryanair said the union should continue to engage with the mediation process and not threaten customers with potential disruption.
Earlier this week, Ryanair's UK-based pilots who are members of the British pilots' union voted to strike over two days later this month and for three days in September.
The members of the British Airline Pilots' Association (BALPA) plan to strike for 48 hours beginning on 22 August and 72 hours from 2 September as they pursue a separate pay claim.
In response, Ryanair said the move had the support of less than 30% of Ryanair's UK pilots.
Members of Ryanair's Portuguese cabin crew trade union are also set to go on strike for five days from 21 August in a dispute over leave.
Ryanair suffered a series of damaging strikes last year, which cost the airline €120m in refunds, fare cuts and lost business.
Last month, the airline warned that it would have to cut up to 900 pilot and cabin crew positions as a result of a number of challenges facing the airline that would impact on its growth plans for winter of this year and also next year.
These include delays in the delivery of Boeing's 737 Max, Brexit uncertainty and a drop in revenues.
Chief Executive Michael O'Leary said the difficulties would result in some base cuts and closures, as well as some job losses.
On Wednesday, Portuguese cabin crew union SNPVAC said Ryanair had decided to close its base at Faro International Airport in southern Portugal from next year with the loss of at least 120 jobs.
In July, the airline reported a 21% drop in quarterly profit as overcapacity and a price war in Germany drove ticket prices lower.
It reported a profit after tax of €243m for the three months to 30 June, down from €309m a year earlier.