This is a refusal to do overtime as I understand it, as opposed to strike action.
However, according to the IAA statment on their website:
Air traffic controllers have committed under successive national agreements to continued adaptation and flexibility. The overtime arrangement that the controllers are refusing to operate is, and has been for many years, part of that flexibility and was not raised by their union as part of its claim during any stage of the Labour Court process. This only emerged as an issue after the Labour Court Recommendation
So, has there been an unwritten rule as such that controllers should do whatever overtime that was required in the past? Or an official, stated agreement?
If it's the former, then the IAA haven't a leg to stand on.