A working hunter competition normally comprises of 3 phases: jumping, ridden (by the judge) and confirmation (short in-hand individual show).
The jumping phase involves a course of approximately 8 fences, normally consisting of one double, one related distance (about 4-5 strides) while the other 5 fences are normally 'island' fences. The fences normally vary between 90cm and 1m, sometimes 1.05m.
The best way to prepare for this would be to do a few show jumping competitions, but remember that the fences used for a working hunter normally don't have coloured poles. Rustic poles and gates are most common, while straw bales, greenery, etc. are normally used as fillers instead.
The ridden phase is where the judge or judges ride your horse on the flat (i.e. no jumping) and test them on how they ride - responsiveness to hand and leg, general way of going, etc.
Obviously flat work at home will help with this. And if you have someone else who you trust to ride your horse, you could let them up for a few minutes and see how your horse goes for them. Not only will it give you a chance to get some feedback from an 'outsider', but it will also give you an opportunity to see somebody else riding your horse and be able to look at his/her way of going.
The confirmation phase requires you to take off the saddle and show the horse 'in hand'. You'll be required to get your horse to stand while the judge(s) look at your horse's confirmation. They'll then ask you to walk your horse away from them and trot him/her back towards them. They'll step out of the way when you get close, so you can continue trotting past them so they get a look at the horse's action.
Again, this preparation can be done at home. Ensuring that your horse will trot in-hand when you ask is really important. There's nothing worse than trying to convince an unwilling horse to trot at any time, especially when you're trying to show it!
Most of the training can be done at home, and having a few rounds of showjumping under your belt would really be the only expense you'd have to incur (just to give the horse experience before show day).
Regarding information, you could contact the Irish Shows Association - www.irishshowsassociation.ie
- I'm sure they'd be able to give you information on rules, etc. as well as giving some advice. Their website also contains a list of the upcoming shows and the show secretaries (it's not up just yet, but should be up on the site quite soon) so you can then contact the individual shows and ask them to send you out a programme with all the classes and entry forms.
If you want some idea of the correct 'turn out' there are quite a few photos in the 'Photos Thread' of showing, so you can have a look there.