AlterEgo
Veteran Member
A border controls both.
Actually the Irish border pre-EU didn't.
The UK and Ireland have a Common Travel Area. The customs posts were not tinstalled to prevent people from coming into the Republic, or the UK, without proper documentation.
They were only there to enforce customs rules. A person walking up the border with no bag or anything in tow would likely have been waved through.
However, that's a moot point as nearly all customs posts are in very rural areas and the overwhelming majority of traffic is in vehicles, which, as you point out, were routinely stopped.
A more interesting sub-plot is that the border actually bisects a vast amount of rural land. Some border farms are owned by paramilitaries or former paramilitaries.