I repeat, what are they to do? They can't leave a registered letter unless it's signed for, and they can't force anyone to sign for it.
The point of the registered letter service is not to provide a guarantee that it will be delivered, still less that anyone will be forced to accept it. It is to provide a guarantee that, if it is delivered, there will be proof of delivery and, if it isn't, (a) you'll know why and (b) you'll get it back. I think the only thing you can complain about is that you didn't get it back.
You know that the reason given for not accepting the letter is bogus, and An Post almost certainly know that too, but so what? It's the reason that was given; they have a responsibility to tell you that; they discharged that responsibility.
The point of the registered letter service is not to provide a guarantee that it will be delivered, still less that anyone will be forced to accept it. It is to provide a guarantee that, if it is delivered, there will be proof of delivery and, if it isn't, (a) you'll know why and (b) you'll get it back. I think the only thing you can complain about is that you didn't get it back.
You know that the reason given for not accepting the letter is bogus, and An Post almost certainly know that too, but so what? It's the reason that was given; they have a responsibility to tell you that; they discharged that responsibility.