My 10 year old son and I are due to fly to Ireland in 10 days time. He is a dual US/Irish citizen. I just realized his US passport is due to expire in 2 weeks time. Can he leave the US on just his Irish passport? We are spending an extended period in Ireland so we can have his US passport renewed before his return to the US. I'm just concerned with leaving the country on a non-US passport. Will I encounter any problems?
Thanks to 8 USC 1185(b) it is
unlawful for any citizen of the United States to depart from or enter, or attempt to depart from or enter, the United States unless he bears a valid United States passport.
(There are exceptions, but they mostly cover things like entry by land, or by sea from nearby islands -- see 8 CFR 235.1(b) if you want to know the details).
The US has no exit controls, one often hears, but it is more precise to say that the US has no systematic exit controls. They do perform spot checks, though these are very rare. Still, there is a very small chance that you could encounter one. If you do, the immigration officer will probably prevent your son from boarding the plane.
Accordingly, the safer option would be to take your son to Ireland with his valid US passport and renew it in Ireland, or, if you intend to return to the US before the end of the year, use the expired passport to return (related: Can a US citizen fly to the US with an expired US passport?).
Another option is to try to apply for an expedited passport using the in-person service for those with travel within 72 hours, but as explained above it should not be necessary. Furthermore, the state department says that appointment availability is "extremely limited," so it might not even be possible.
EDIT: I was mistaken. U.S. law requires you to use a U.S. passport to leave or enter the US even if you have a dual citizenship.
That being said, your US passport is stil valid at the time of your exit, and you already have plans to renew it in Ireland before reentry, so there shouldn't be any issues.
There is no difference between leaving the US using a US passport or an Irish one. There are no controls when you exit the country. Considering you're traveling toward the EU, using a European passport is probably convenient anyway in the unlikely scenario any issue arises.
The US, like many other countries, only has controls on entrance. That's where the difference comes into play, making the process much faster for US citizens over anyone using a foreign passport. But if you'll have his US passport renewed before then, you shouldn't have any issue on reentry.