Upvote:1
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:
https://www.iatatravelcentre.com/passport-visa-health-travel-document-requirements.htm