SIPTU has served notice of industrial action on Aer Lingus in a row over a €700 million pension scheme deficit.
SIPTU members at the airline are due to hold work stoppages on Monday 1 October.
SIPTU aviation sector organiser Dermot O’Loughlin said notice was being served because of what he called the airline's decision to renege on its obligations.
SIPTU served similar notice of industrial action on the Dublin Airport Authority two days ago.
Both companies contribute to the Irish Aviation Superannuation scheme and have been unable to resolve the row over how the deficit should be addressed.
Aer Lingus said it regretted that strike notice had been served and refuted the union's claim that a breach of collective agreements had occurred.
The airline said it would assess the situation over the coming days and would communicate with customers over alternative travel arrangements in the event of any likely disruption.