Instructions:This code shared on the cityjet twitter supposedly for flights to/from london city. However it seems to work on other flights/routes too so worth trying. Only valid for flights departing before 3rd May 2015. Not sure of expiry date.
Book any @cityjet flight to @LondonCityAir for departure before 3 May 2015 and get 10% off by quoting 'LEINSTER'