A number of UK universities have their policies on the appointment of Honorary positions on their websites. It appears from a number of them that the appointments are usually made upon the request from for example, the head of a school, within the institution.
It appears (from the University of St Andrews) that appointments are made when it is deemed to be beneficial to both parties.
An Honorary appointment is an arrangement which allows distinguished individuals from other institutions and organisations to collaborate with academic staff at the University of St Andrews and have access to certain University facilities to further their own research, cooperate on joint research or contribute to the teaching of the University. It recognises an ongoing association with the University.
In all of the policies it appears that the Head of School makes that application and it is the HR department that makes the final decision on the granting of the Honorary position.
I relation to gaining a Honorary position, I would think that if you are well known in your field of research there is a better chance of gaining one.
Here is a list of links to policies for the following universities.
- University of St Andrews
- Swansea University
In Ireland (my home country) I believe they are occasionally awarded. Trinity College Dublin have Honorary positions in their listing of academic titles