Emerald Airlines pilots have started voting on industrial action.
The ballot, which will run until May 18, is "in response to an ongoing trade dispute with the airline" and could lead to the pilots going on strike.
The pilots are members of IALPA, the professional association which represents pilots within Ireland, and a branch of Ireland's Fórsa trade union.
IALPA principal officer Owen Kelly said the airline "does not recognise the trade union as the representative negotiating body for its pilots".
“Emerald’s position suggests it’s currently out of touch on how best to sustainably establish terms and conditions for its employees in a highly competitive labour market. This is why this ballot became necessary,” said IALPA's David Langan.
Emerald is the regional operator for Aer Lingus, the Irish flag carrier that is part of International Airlines Group but for decades was a state-run business. The union said among Emerald's operations are "the public service obligation Dublin to Donegal service".