Got mine yesterday using Parcel Motel to get round the delivery problem to Ireland. Same Error Code 5505. This is the only way I can see at the moment to get round this. Download the Prime Video app onto an android device. I tried my iPhone but it never worked.
Press what you want to watch on your android device. It gives you the option in a green box to ‘Watch on TV’. Click that and it casts to your TV, you can then use your Fire remote to pause/play etc. Not ideal, but it sort of works.
It was the same for me, but today I contacted Amazon support through help page -> contact us -> devices -> fire stick -> prime video.
I was chatting with the support guy, told him I am in Ireland and I cannot watch on the Fire stick the same content I am watching on other devices using Prime Video application.
He changed a regional setting @ their end and now Prime video works on fire stick.
I think the simple answer to your last question is that Amazon have made it so.
That is also perhaps why they do not sell the Firestick to addresses outside the UK.
Just because you "expected" to be able to use the Firestick for Prime Video access in ROI despite several indications that it does not work for this outside the UK, does not mean that there is something wrong, other than perhaps with your expectations.
Someone on a Reddit forum advised to contact Amazon support and they will add the correct regional settings (different from the settings available in the account).
Amazon support sorted it out, as "expected" (after all it was working on other devices).