I have an Irish passport so I don’t need a visa.
Do the US officials come on the train and see your passport and just go to the next person?

The fact that you are showing a non US/Canada passport would lead them to be more alert.

You still need a ESTA approval, which you don't have based on your previous question.

• visas - If one of my passports has been denied ESTA approval, can I use my other passport to apply for ESTA? - Travel Stack Exchange

Sources:

• ESTA - Do I need an ESTA to enter the United States by land?

Effective October 1, 2022, all Visa Waiver Program (VWP) travelers intending to enter the United States by land will be required to obtain an approved Electronic System for Travel Authorization (ESTA) prior to application for admission at land border ports of entry.

CBP encourages travelers to apply for a travel authorization at least 72 hours in advance of their anticipated arrival at a U.S. land port of entry.

I have an Irish passport so I don’t need a visa.
Do the US officials come on the train and see your passport and just go to the next person?

The fact that you are showing a non US/Canada passport would lead them to be more alert.

You still need a ESTA approval, which you don't have based on your previous question:

• visas - If one of my passports has been denied ESTA approval, can I use my other passport to apply for ESTA? - Travel Stack Exchange

Comment from @phoog: With the ESTA denial, a visa will be necessary.

Sources:

• ESTA - Do I need an ESTA to enter the United States by land?

Effective October 1, 2022, all Visa Waiver Program (VWP) travelers intending to enter the United States by land will be required to obtain an approved Electronic System for Travel Authorization (ESTA) prior to application for admission at land border ports of entry.

CBP encourages travelers to apply for a travel authorization at least 72 hours in advance of their anticipated arrival at a U.S. land port of entry.