"Stripping out US focused transit flights to places East what would the decline be ? "
Old women have gone to prison over here trying to stop these flight. Another matter entirely.
But your right. Strip out the transits and Shannon is in trouble. Strip out anything that is worth mentioning and Shannon is in trouble. Shannon like any other airport needs all the business it can get and even then will struggle. The big picture is that the terminal traffic is up. Transit traffic is volatile but has been a significant part of SNN business since it opened, the reason it opened in the first place.
Loss of SANTA flight ie: 0.0065% of non revenue business in 2014 will make no difference what so ever except that domestic traffic stats will be down about 90%. 
My parting comment on this matter is how sad and sickened I am at the level of cynicism levelled at the Shannon airport authority by fellow so called enthusiasts.