Couldn't anyone illegally entering the country do the same ? Nobody checked our passports, the dogs animal health certificates, anything - they just checked our vehicle had a ticket and waved us through. Anyone illegally entering the country could just drive onto the ferry, go into the Lounge for the crossing, and then drive off the ferry at the other end ?
This is the compromise made to support the Good Friday Agreement, and Brexit.
After Brexit, there should have been a
hard border between Eire (an EU country) and the UK. This would have prejudiced the G.F.A. which had basically removed all border controls between the North and South parts of Ireland. (The G.F.A. has always been seen by Americans, as the first visible step toward unification.)
In principle, there ought to be a border somewhere in the Irish sea. This wasn't acceptable to a loyalist faction in the North, who felt any border between the mainland and the North was not acceptable. There is no solution to three parties (Americans, EU, Loyalists) all wanting different things. The British government didn't and doesn't have a choice that involves border controls. The answer was if the EU wants a border, they can pay for it and police it. The Irish, who are EU members obviously, want to keep the G.F.A. in place, so their "border" is more a concept than a reality.
The demographics of N.I. are changing and the anticipation is the Loyalists will no longer have a majority vote in 5 to 10 years, "in theory" enabling a referendum that could lead to unification. Why "in theory"? The people in N.I. currently enjoy support from the mainland that includes funding for a 'free at point of use' healthcare service. And many other 'benefits' that make living in the UK, attractive to those that don't live here. It's possible that even if there is a political will for unification, there maybe be some financial reluctance by those who might otherwise cast a winning vote in favour of unification.