He was still with Ferrari as consultant to be honest. But with completely different package coming up most top teams will have 2 different teams working at same time on different projects. Makes sense to pull in Rory for more hands on approach for 2014 with the other team focused on 2013 car.
A fundamentally good design is fundamentally good design. Ferrari can develop that over season. Rory has incredible track record. As of now it looks like he is going to work as hands on team advisor rather than lead designer which is still going to be Nikolas Tombazis.
It is relatively very young design team at Ferrari at the moment. The 3 key people (Fabio Montecchi, Simone Resta and Nikolas ) at the top are very young and with less experience than likes of Newey. So added hands on input from someone like Byrne will be very useful.
Nikolas is actually one of the better guys... he left Ferrari and did a stint at McLaren for a while. he was much hot property in early 00's.