Passenger spend down 30% and the airport has cut it's cost base by the same amount according to an interview with the airport Chairman last month:
Passengers set to flock via Knock - The Irish Times - Fri, Mar 25, 2011
Not clear what spending he's referring to or what period. Several factors could be at play: online check-in cutting down time in terminal, less discretionary shopping, tight limits on carry on liquids, LOCO restrictions on carry on baggage, relatively basic retail and food offering. Bus services could have had a small impact on parking.
And the unique Knock phenomenon of relatives dropping off pax and having a few drinks in the viewing lounge to wave them off is also dying out as the airport gets more commercial.