Miguel Cruz wrote:
Jean S. Barto wrote:
Thanks for the advice, everyone--guess I'll have to carry both passports
with me during my trip, and be prepared to present my US passport on the
train as required--and use the Irish passport at passport control (whichever
ones are left) as required.

You must have your real passport with you anyway, because if you are a US
citizen (even if you are also a citizen of some other country) holding a US
passport it is illegal to enter the US by showing any other passport.

"Real" passport. Irish passports are real passports, too. You're
correct that the holder must have a US passport in order to return to
the US, but you could have probably used a better word other than
"real".