"Legal Tender" is just a legal term. People get very hung up on whether or not a certain object is "legal tender" or not and the common perception of the definition is VERY different to what the legal definition is.
Very few objects are legally defined as "legal tender" and the wikipedia extract (although potentially spurious given the source I know) is pretty much legally correct.
However as others have said the notion of "legal tender" is a bit of a nonsense and anyone can accept any item as payment as they see fit.
In this case the Northern Irish notes should really have been accepted. If an airline doesn't want to take payment for an additional set of goods or services more fool them!