Getting into Canada on an Irish passport is fine and correct.

Have you tried calling CBP and asking? Tell them that you are in Canada, and that, in fact, you have a passport, but not your US passport, and how you should proceed. I don't think it should matter if you forgot the passport at home or if it is stuck at some embassy.

For future reference, it would be interesting to see the actual rules of how CBP staff is supposed to deal with a US citizen who presents a different passport at an entry point on an airport. A Freedom of Information request could tell.