There are several agencies
www.aupairireland.com being one, try
www.rollercoaster.ie also, maybe start a thread on their discussion forum. Agencies will obviously charge a fee, whereas the language colleges don't
We have one year old twins and a three year old, and after the twins arrived, things got a bit hectic at home to say the least, so we decided to get an au pair so Mrs Mac could retain some degree of sanity. We went direct to one of the Dublin based English language schools, all of whom have regular intake of students on 3 -12 months assignments. I actually can't recall which school it was ( Mrs Mac looked after it), but you'll find a list of them in Golden Pages. We initially took one on three mths, and she has ended up staying on with us after her course finished.
We pay her €80 euro per week, but don't be fooled into thinking this is cheap. You asked for an example of some issues so let me give you example of some cost issues. I've never seen any adult human being eat so much food, and it in fact it is becoming an issue for us. I'm not being tight here, but she yesterday mananaged to put away 10 pieces of fruit (yes, prety much the whole fruit bowl )on top of multiple snacks (yoghurts, ham, the kids chocolate buttons, bowls of cereal )on top of her three square meals (all of which were substantial meals). Every time she goes out, she takes food with her, also stashing fruit/biscuits/snacks in her room. Passed a comment to her last night about her food consumption and she just laughed. I can't afford her eating habits, and despite her being very good with the kids, she is eating me out of house and home. She showers twice a day, and boils the kettle at least ten times a day. She asked if it OK if she took on a part time job, which we said fine with us, so long as not interfering with her primary duties to us. This has escalated to a five half days per week, so inflexibility has crept in. She does 30-35 hours a week for us, mostly just watching the kids to be honest (tough job I know), rarely anything physical. Mrs Mac spends half her free time going to Tesco to top up the fridge/larder to keep the up with the Au Pairs eating frenzy. She will be going home in the near future if she doesn't start to show some restraint.
Of course the most important thjing is that you can place your trust in the person to responsibly look after your children. Nonetheless, beware the hidden costs, they can really stack up, and make the whole thing financially unviable.
Good luck.