SIPTU says it has 'no objection' to working with Defence Forces members at Dublin Airport.
The union says its representatives have been given assurances around the activation of a contingency plan for the airport.
It says their members will not be asked, or required, to work alongside Defence Force members or to train them.
SIPTU aviation sector organiser Niall Phillips told Newstalk Breakfast their members raised some concerns.
"Our members have no objection to working alongside Defence Force members.
"There were expressed concerns in relation to reporting and responsibilities to around manning the VCP posts.
"What we're talking about here is in the event of the contingency plan being activated, that the Army would be deployed to what's known as the Vehicle Control Posts.
"These aren't public-facing roles within the airport, but these are posts around the perimeters of the airfield.
"But our members, again, did not refuse to work alongside the Army because we never got to that situation.
"We raised concerns on behalf of our members with the DAA.
"And the DAA assured us that our members wouldn't be asked to work alongside the Army - nor would they envisage it that in the event of the contingency plan being triggered, our members from the vehicle posts would be moved into the terminals to support screen of passengers".
But he says there were some concerns over "different reporting responsibilities, and who would have responsibilities then at the posts".
"We don't have an issue with the contingency plan, but we needed clarifications and certain commitments around our members - and what would happen then in the event of this contingency plan being activated", he adds.
In a statement, the DAA said over 420,000 passengers have travelled through Dublin Airport since Friday.
"We are keen to avoid capacity reductions, which have been deployed at other UK and European airports, as these would have a material impact on the summer travel plans of a significant number of Irish passengers and families most of whom have not enjoyed a foreign holiday in over two years.
"There are risks associated with a new summer wave of COVID-19, so it is important that contingencies are in place to safeguard the operation throughout the summer period.
"DAA is facilitating the training of Defence Forces personnel who would only be deployed to non-public duties at the airport's perimeter Vehicle Control Posts, should this be required in the event of a COVID spike."