I can't see the government threatening to block the EU constitution on this one. In fact if the EU wasn't involved, I can't imagine them trying to amend the bilateral. Although they can see the economic sense in dropping the stopover, they are far too worried about vote loss in Clare, Limerick East and Tipp North and want to present this as a decision that has been forced by Brussels.
In 9 years of being Taoiseach, I can never remember Bertie Ahern sticking his neck out over an issue. I wouldn't hold out much hope of him setting a precedent here.