It´s not easy. Even to get a position as a volunteer can be a problem. Last time they were hiring volunteers they had 300 applicants fo 30 position. And even if you become a volunteer, it won´t guarantee you a job in future.
I was talking with one Dublin Zoo employee recently and she said that the situation changed rapidly over the past few years. Before, there was nobody with proper qualification and people simply worked they way up. But now, with all the schools and courses available, there is a lot of qualified people to do that, so the competition is strong.
So a course is not enough either. You would need not only qualification and experience, but also something "extra", some kind of expertise they don´t have at the moment.
But don´t give up! Even if you won´t get to Dublin Zoo, there are other things you can do as an animal lover!