Im not sure if the air travel tax is charged on inbound flights, or just the outbound?
Either way I'm not sure that €3/6 is going to make such a difference that 300,000 extra seats can be filled. Are passengers really that price sensitive. Or is is €3 extra in the pocket of FR on every seat they sell ex Ireland? Essentially an extra €1 million annually for each based aircraft.
The routes ex SNN appear mainly to cater for Diaspora to Eastern Europe and Sun Seekers, very little inbound tourist potential outside the Munich, Berlin and maybe Paris routes.