The main difference with the Canadian approach over customs is that to do them in Ireland, which might be as practical as doing immigration, would prevent on the transatlantic flight the duty free drink on board and the duty free goods sales, which make the airlines revenue and make the passengers happy.

Furthermore, if immigration refuse you entry to the US that's just too bad and you don't leave Ireland. If customs detect you are seriously smuggling, failing to declare things, or whatever, they could not extract any penalty in Ireland for that. Between Canada and the US there just aren't the differences in the same way.