Transport Scotland confirmed it had an agreement in principle in place with Stair Estates to use Castle Kennedy Airfield for overflow parking if traffic heading from Scotland to Northern Ireland backs up.
The Operation Overflow plan aims to assist if ferry operators at the port cannot cope with disruption caused when the Brexit transition period ends on January 1.
A Transport Scotland spokeswoman said: "While we still need clarity from the UK Government on the terms of exit, including the requirements for checks on goods moving from GB to NI, at this stage we do not expect a significant increase in traffic or delays at Cairnryan.
"Nevertheless we have been engaging with ferry operators who have assured us that they have capacity to cover increased demand on the Cairnryan/Larne routes.
"We have worked with local resilience partners to develop a contingency plan in case of traffic disruption at Cairnryan and are now moving to test that plan."
The transport agency said discussions continue with Stair Estates on the plan.
The House of Commons rises for Christmas at the close of Thursday's business but MPs have been put on standby to be recalled if a trade deal is secured.
Brexit negotiations were given the green light to continue on Sunday following a meeting between Prime Minister Boris Johnson and European Commission president Ursula von der Leyen.