My Mr and I got our GE at the same time; he is US non-resident alien so not even a US citizen. We were interviewed by the same officer but one after the other and not together. He was asked the same generic questions that I was, nothing even remotely memorable. Interviews took about equally long (maybe 5 minutes of questions then take fingerprints and such). I would guess that if a UK citizen gets the same questions that a US citizen does, then a dual US/Irish citizen would be more or less in the ballpark.
If you have two passports, or a passport and a green card, they are quite particular about which document the GE encoding is placed on. For a while it would work on the Mr's passport as well as green card; then he started getting email nastygrams when using the UK passport, and now the passport doesn't work at all, and he must use his greencard.