This is false. If you fly into Dublin from Bristol then you do go through passport control.
Mostly true, but there are complications. Were you say, on a Syrian passport, flying from Dublin to Bristol you would not qualify for FTA. This is why there exists passport control at each end.
I am unsure why some people don't get that.
One may freely travel from the UK to Ireland, or vice versa, so long as you are a citizen of either country. This does not, obviously, apply to a non Irish/UK citizen. Thus, one must present evidence that one is an Irish/UK citizen. Hence passport control applies. Hence an passport from explodistan might attract attention.
It is a bilateral agreement. See that "bi" at the front? That means two.
Those two would be the UK/Ireland. It does not apply to anybody outside those two.
I cannot figure out why anyone thinks that a bilateral agreement applies to others beyond the two who made the original agreement, nor how the boogey man will sneak in on foot of it.
Now consider what happens at either end. One gets a plane load of people. What does one do? Well, one checks. Are you actually British/Irish? No? Let us have a chat.
And that happens exactly how? Passport control. This is obvious. This is what happens. Demonstrably Irish? In you come. Demonstrably British? In you come.
Neither? OK, wait a minute, further questions may happen.