Not gonna happen, Croke Park is more like 85,000 capacity btw.
If GAA officials somehow (highly unlikely) allowed the UFC to hold an event at Croke Park there is absolutely zero chance UFC would be allowed hold the main event late around 4 or 5am to cater to the American audiences, like they're doing for 204.
Also, Ireland isn't exactly USA or Brazil when it comes to the weather so good luck picking a day when it's not pissing rain. There's a reason Ireland is so green...