Airbourne.
You also have to go through security at Atlanta, again for the reason that you have to go through the "sterile" area on arrival. This also means checking in and reclaiming hold baggage.
Certainly flights from several Canadian airports are deemed domestic and arrive at domestic concourses. I don't know whether the TSA operate security at Canadian airports or audit it. (Remember that TSA does not operate security at SFO). I would suspect that DUB/SNN will be similar to Canada.
You would think that you could get agreements between different countries to recognise each other's security, as is happening in much of Europe, but remember that the UK requires transfer passengers (from outside the UK) to be rescreened as it does not consider other European county's security to be up to UK standards so it is not just a US issue.
When I was at HKG passengers to Australia & the USA (but not the UK) has to undergo secondary security at the gate. I'm not sure of the logic of it all. I presume that different countries audit each other's security arrangements.