Advanced Passenger Information and Citizenship

Hi, not strictly VS related, but not sure where else to put it, but do need a guru's advice!

Trying to fill in my friends API for our Vegas trip in a few weeks time, but not sure on his Citizenship [:?]

He has an Irish passport, with an Irish mother and a British father. Lives in Britain (own house, voting etc)

Now he's not sure what his citizenship category answer should be.

The ESTA application does not ask for citizenship, just passport issuing country.

Any help would be appreciated.

The reason there are two county codes for APIS is because one is for the nationality of the document that they are travelling on and the other is for the nationality of the passenger.

So if it is a passport they are both the same.

If you are a Japanese person travelling on a UK ID card the document is GBR but the pax is JPN.

Citizenship is not the same as residency.

Thanks!