Entering Spain with Soon-expiring US passport and expired Irish passport

Ryanair has a very strict document check before they would even let your friend board the plane. I would very much assume them to recognize that the US passport is not valid long enough and not let him/her fly. There is no reason to expect that Ryanair nor the Spanish immigration police will accept an expired Irish passport as proof for Irish citizenship.

Since the passport office in Dublin offers an urgent appointment service with same day renewal, it is hard to believe that there is no time to renew the Irish passport. I would guess that is the only way to be able to travel.

You say that his US passport is expiring a few days after his return flight, so he won't be able to travel with his US passport on Ryanair, and of course not with his expired EU passport either.

From Ryanair's General Terms and conditions :

2.1 Photo ID needed for travelling abroad

2.1.1 A valid passport

2.1.1.3 All non-EU passport holders travelling into a Schengen member country must make sure that their passport is valid for at least three months from the date they will leave the Schengen member country, unless the person has a Schengen-issued residence permit or long-term visa.

It goes without saying that an expired passport is not a valid passport. Also, note that Ireland is EU, but not Schengen.

However, he would be able to travel with an Irish Passport Card, provided he has a valid one:

2.1.3 An Irish Passport Card (only for Irish citizens travelling within the European Union and the European Economic Area)

.. which is unlikely, as a passport card is valid for 5 years or until your passport book expires, whichever period is shorter. It can be assumed that if his Irish passport is expired, his Irish passport card is expired too.

Also, interestingly, you ask:

The requirement to have a passport valid for 3 months after return does not usually apply to EU citizens, but does it apply to EU dual citizens travelling on their non-EU passport?

Apart from the fact that your friend would be unable to prove that he still is an EU citizen, considering that his EU passport is expired, hence invalid, once you choose to travel on a passport, you travel on that passport and as a citizen of that country. Generally speaking, you can't mix and match to circumvent requirements and limitations.

Irish passports online renewals are very fast atm. Mine came in days rather than weeks/months. Unlikely to beat the evil barstewards at Ryanair. I have to travel with them next month (lack of alternatives), but I normally go out of my way, literally, to avoid travelling with them.

There isn't time to renew the Irish passport. Is there anything he can do?

If there truly isn't time to renew the passport, even taking into account the urgent passport service, and assuming that the traveler doesn't have an Irish passport card, there are three options:

1. Give up on the trip; rebook if possible or write off the tickets if it isn't possible, or

2. Try to fly with the valid US passport and the expired Irish passport, recognizing that failure is likely, as outlined in Tor-Einar Jarnbjo's answer.

3. Modify your itinerary to enter the Schengen area by rail. The existence of preclearance in the UK means that the rail company does not enforce passport requirements, so all you need to do is to convince the French border guards there to admit you.

If the only alternative is to lose the value of the tickets, and if the traveler is very persuasive, there might be a small chance of getting through. I wouldn't count on it, though.