Unfortunately they'd certainly be registered in Ireland and Irish Privacy agency doesn't hand out notable fines ...
Enforcement of GDPR is a different matter. I am not sure whom I'd complain to, other than my representative in the European parliament. I could probably sue them though, if I found my data there and they refused to comply with my GDPR request.
No. To give a counterexample, there are EU citizens in China, and most internet companies in China don't care a fvck about GDPR, especially Tencent. You bet your conversations on WeChat are logged, sometimes subjected to censorship, and more, it's just how things work. Heck, there are even laws in China that are fully contradictory to GDPR, and the local laws take precedence, regardless of what citizenship you have, if you are physically within the borders you are subject to local laws.
The US isn't any different from China here. The EU doesn't get to enforce its laws in China, and it doesn't get to enforce its laws in the US. They aren't the world police.
That said, I'm not saying GDPR is bad, just that the US needs to have its own version of it in US law. What is bad is letting the EU enforce laws outside their borders. China doesn't, and neither should the US.
Companies outside of the EU can become subject to GDPR by targeting EU residents. You can always raise a complaint to the data privacy regulator for the EU country where you currently reside, and GDPR has a mechanism to forward that to the relevant authority.
There are good parts in GDPR for sure.