I am a US Citizen, and I need to travel to Spain in a few days but my passport is recently expired. I have dual citizenship to Ireland and hold a valid Irish passport that states my birthplace is the US on it. Would I be able to travel on this Irish passport while also brining my expired one with me as proof. I read that as of last month you can travel back into the United States on a expired passport until December. Will this work for me?
This should work. You should be able to get into Spain with your Irish passport.
You can return to the US with an expired passport if you return before the end of the year. https://www.state.gov/return-travel-on-recently-expired-u-s-passports/
Before you book: I strongly recommend that you contact the airline and verify that they will let you board the return flight with an expired US passport. Otherwise try a different airline.
To verify put all your info in here: