I came across a six year summary of passenger statistics for Shannon (apologies on the poor formatting), just to illuminate that question of "where did the passengers go?"
2012 2011 2010 2009 2008 2007
Europe 236,118 269,718 323,358 888,946 1,020,437 1,066,630
Other
International 581 745 1,063 869 2,820 507
Domestic 1,393 1,649 25,064 52,448 96,836 89,034
Transit 132,799 280,582 295,230 347,238 334,623 464,331
Total 1,394,781 1,625,453 1,755,885 2,794,563 3,169,529 3,620,623
The key components of the 2 million plus fall in numbers seems to be about 400,000 Transatlantic, 500,000 Britain, 750,000 Europe and 300,000 transit.