Kilter, check the public jobs website, it will have examples of the preparatory material.
Also, don't be put off by the numbers going for the exam. Generally, people with secretarial/office courses apply for CO jobs (with the exception of a few ppl like myself who want less stress). These are mainly people who got a pass leaving cert, maybe a few honours who didn't go to college. Therefore the standard of applicants is lower than the standard applying for the Executive Officer, Third Secretary. I think that if you are reasonably intelligent & have a degree, you should have a good chance of being placed in the top 20 anyway. Go and do the exam & worry about the rest later.