It is impossible to use the March, April or May figures to compare this year against last.
March figures will appear lower because of the late Easter in 2011 (so well done to Derry for showing an increase this year in March).
April figures will be massively higher because of the late Easter coupled with the ash cloud last year.
May's won't be reliable either because the airport closures in 2010 extended well into May and also people who couldn't fly outbound as planned in late April didn't return in May as a result of their trips being cancelled before they even left.
So expect big gains across the board for airports and airlines in April and May and a truer picture to emerge from June onwards.