The advertisement states its for future careers in Aer Lingus and that ''We are currently looking to develop a panel of suitable candidates which may be progressed to the initial stage of the selection process should positions become available in our In-flight Services Department in Dublin''.
That may mean you may only have an interview when there is positions to fill, or they may interview you and put you on a panel.
Contracts are temporary, most likely for 6 months.