The data protection law exists for the protection of personal data, defined as "data relating to a living individual who is or can be identified either from the data or from the data in conjunction with other information that is in, or is likely to come into, the possession of the data controller"[1]
If the airport authority is not gathering any other information beyond the device MAC address, and is not connecting that information to other information sources (such as MAC addresses gathered from WiFi registrations) then the MAC address cannot be used to identify a living individual.
Now, we all know that there is a correspondence between such a MAC address and a single individual, just as there is with DNA or fingerprints or footfall pattern or voice analysis or facial recognition, or car registration numbers or any of a number of other identifying characteristics. However, the law does not concern the gathering of data (however unique), merely the gathering of personal data that identifies someone in conjunction with other data in the possession of the data controller.
Just because that data could be used to identify someone (in a different system) does not mean that it is covered under data protection obligations in this system.
[1] https://www.dataprotection.ie/docs/What-is-Personal-Data-/21...