It's very easy in Ireland to work for top riders as a working student. As awful as it is to say, staff to a lot of them (not all, but a lot) are disposable and because of this, have a large staff turn over. With a large staff turnover, they're always looking to replace staff. I would strongly advise going for a month or two, then asking to stay longer if you enjoy it. The reason for this is at least you leave with a good reference and you haven't bailed out as such. They are considerably more picky about who gets on their horse though.
As R p says though, it's better to start small and work your way up. You haven't mentioned if you've worked in a yard outside of a riding school before, but if you haven't, I'd start getting work once or twice a week in a small yard first. Perhaps a livery or a breaking/schooling yard to see and ride a wide variety of horses. Better you get some experience before looking to go into bigger work, than start higher up and have to be taught everything. The higher up you go in the horse world, the smaller the circle gets. Nearly everyone knows everyone else so there's a good chance a potential new employer will know if you're coming from an old employer as a novice, which they may or may not be bothered about.
As for riding, get lessons. Get good lessons. Ask around and look for well recommended instructors, especially in flat work and dressage. Often grooms are asked to do flatwork and hacking, and if you can prove you can ride well on the flat, there's a better chance you'll be asked to do jumping work too at some stage. However, if you don't ride well on the flat, there's no real hope of you being asked to jump them.
I'd probably suggest asking around, calling stables/farms where they're based and asking if they want someone to work, and/or if not, or if you're not suitable, ask what level of experience is required to work there, then keep ringing up people/stables that are less and less of high status and start working where you can to get your knowledge and experience up. Then eventually you'll be good enough and have enough references to work at a top class places.