How to Invoke Common Travel Area on Arrival

[billabong1]

Hello,

As freshly minted British (me) and Irish (my wife) citizens, we're moving in the new year to the UK from the US. I understand that as a British citizen I can just walk in, but for my wife to use the benefits of the CTA, should she announce that she is immigrating to the UK, or just enter? What experience has anyone had with trying to get a National Insurance number? I tried on a visit earlier this month and was told that if I wasn't working or applying for benefits, I had no need for an NI number. Has this been other's experience? Thanks for any help.