Nav 1, this information may be a little out of date, as I sat an interview with them a few years ago...
The preliminary interview is fairly easy. Two people on the panel, they go through your details (college, school, work, home, interests etc), they also give you some simple questions on directions, basic maths etc.
After that it's a computer based aptitude test which is tough enough (accuracy & speed being the keywords). I didn't get any further...
By way of comparison, I found the interviews for Aer Lingus much harder, but the aptitude test for Parc/EI was easier...
Bets of luck!