All these Americans are doing is trying to improve their lives and careers.
At the expense of their European colleagues...
There is no way they could know this sentiment truly existed before they signed.
This whole topic was extensively aired here on PPRuNe a few months back, and there were contributions from your side of the pond, so you are wrong there. More to the point, you have to be rather naive to think that a whole bunch of American pilots turning up to fly in Ireland is going to be greeted with great glee by your Irish counterparts.
As others have said, this would never be allowed in the US, so why do you think the Irish pilots are going to be happy about it? How is it that you don't get that you are disadvantaging Irish pilots? If the reverse DID happen in the US, and a flood of Irish pilots disadvantaged you, how would you feel?
Pretty angry, I would think.
You can't just say "it's not the pilots' fault". They knew that they would be having the effect described above - you would have to be pretty foolish not to.