Originally Posted by
ajax
Now, I'm not saying you're wrong, I'm just saying that this is very counter-intuitive.
Northern Island is part of UK sovereign territory and subject to British immigration controls (i.e., if you fly from outside of the British Isles to Belfast directly, you go through British immigration when entering). The ROI has a different set of immigration controls. Doesn't that preclude free movement throughout the entire island?
Ultimately, you can cross over the border from one sovereign nation's territory to another's, but that doesn't mean you're there legally.
You are there legally, as the United Kingdom and Republic of Ireland operate a common travel zone policy. Clear immigration in Dublin, and you are free to travel anywhere on the island of Ireland. Should you want to fly or go by ferry to Britain, you will be subject to the same id verification as you would for a domestic flight in the US.