Earlier quoted context omitted.
"What is more probable is tha FB will try a legal trick claiming they are an American company and they follow the laws of California, not EU, even if they gladly accept European advertisers's money via Ireland" That is specifically not possible under GDPR. It doesn't matter where your company sits, if you store data from europeans (or people living in europe) you have to follow the GDPR with potential for severe puni…
How can the EU punish a business with no EU location? An American company is not bound by another countries laws
They can also file a case in the country that you do belong you to get you to pay your fines in the original country. This sometimes works.