Sixteen more Ryanair flights are at risk of being cancelled next Tuesday as strike disruptions escalate.
A resolution between the Forsa union and the airline does not yet appear to be in sight.
Ryanair pilots will first strike today, affecting 4,000 customers flying between the Republic and the UK, with the airline labelling the action "unnecessary".
Last night it emerged 16 more flights were set to be cancelled by the airline on Tuesday after strike notification from the Forsa union representing Irish pilots.
Ryanair tweeted a letter it had addressed to Forsa, stating the airline "calls on Forsa to call off July 24 strike by less than 25% of our Irish pilots".
The letter addressed to Angela Kirk, national secretary of the union in Dublin, referred to a meeting the airline had with the union on Wednesday.
"While this meeting was primarily to update the new members of your team, nothing has progressed since we have got no engagement or response from the pilots' committee on Ryanair's proposals."
The airline said that unless it hears back from the union "we will issue cancellations tomorrow to passengers on 16 (of over 290) flights to/from Ireland next Tuesday".
It added that it could "readily re-accommodate this small number of customers on other flights", and said: "Forsa's lack of urgency is unacceptable when your committee are unable to engage or negotiate."
But Niall Shanahan, spokesman for Forsa, said: "Ryanair management made no reference to the strike action during our meeting with them yesterday."
The pilots are holding a 24-hour stoppage today and they also plan to go on strike next Tuesday.
They are seeking a master seniority list to be established to give those with the longest tenure the first option to promotions, holidays and base transfers.
Regarding today's action, the airline said 90% of the 4,000 customers affected had been re-accommodated.
Customer service teams were assisting passengers with refunds, free transfers and alternative flights.
Ryanair will also be affected by strikes from cabin crew in Belgium, Portugal and Spain on next Wednesday and Thursday.
Ryanair was asked if any of the 300 cancelled flights each day would affect Irish passengers but did not provide any information.
The airline has said that some 12% of their customers will be affected by these cancellations.