I’ll bite. Why do you think it’s an affront to their sovereignty? It’s entirely voluntary and it’s something the Dublin airport (and the dozens of other airports in Canada) actively seek out to get direct access to the domestic side in the US.
The US does not force any airports into these arrangements.
As said policies become increasingly inhumane I think Ireland should consider removing this arrangement. But you are right, Dublin Airport themselves do benefit since it makes them more attractive, especially as a transfer airport for people going to the US from Europe.
If anything, it seems to me that the USA agreeing to perform immigration checks in Ireland and accept them when you reach the USA is a(n extremely mild) limitation to US sovereignty, not to Irish sovereignty.
(The popularity of that airport for CIA torture flights also doesn't help the case, even if not directly linked)
FWIW, I recall reading that the program in Canada is reciprocal, and it is simply the case that Canada hasn't decided to operate any corresponding facility in the US.
Let's talk about Toronto or Vancouver to set aside CIA whatever. What particular convenience does it provide for the US government to do it there vs on the US side? AFAICT that would save the airline that brought a person who got denied a bit of trouble - vs having to take them back to their departure airport - but not be a particularly huge convenience or burden for either government at a higher-up level.
It reduces legal accountability (I know the US courts have generally exempted border operations from the constitution anyway, but that interpretation could change in the future) and makes it easier to prevent people from e.g. landing and claiming asylum (yes there are measures to penalise airlines and oblige them to return passengers, but they're not always fully effective). More subtly it means there's less pressure to have reasonable border rules, since turning someone away before they board is lower-stakes. And having an official, pseudo-law-enforcement presence in a country is valuable almost in itself.
Well maybe. What happens when you try to exercise that right? If it turns out that those US border officials falsely imprisoned you (under Canadian law - if the Canadian courts are even willing to hear the case), what consequences will they face?