I used to work for an agency a good few years ago, so I can give some advice.
With Agencies, you are paying for the security that the Au Pair placed with you has been screened, vetted, and all relevant checks have been undertaken. The agency will usually have the originals so you know they are legit. Sometimes on the websites this is not the case and some are counterfeit, so be careful from that point of view.
I've seen Au Pairs placed with families from both agencies and websites and in my experience the agencies have the better result in the long run as the running work has been done for you. They've openly asked the prospective Au Pair questions such as where they would like to be placed (city, country, etc.), how much they would like to be paid each week, how many hours they are happy to do, how long they really want to stay for, so many questions its unbelieveable. They do this before they match an Au Pair with any family so that they know exactly what the Au Pairs expectations are and that they meet the families needs and wants.
I've seen on some occasions Au Pairs through the websites are so desperate to get to Ireland they agree to anything and everything with loads of smiles and nods and nice phone calls, and then once here either do a runner once they have a bit of bob, or they then change their minds on things. It happens sometimes with the agencies too, dont get me wrong, but overall not as often as they dont want to match a girl to a family that does not suit and visa versa. An agent will not introduce a girl wanting 2 months in Ireland to a family looking for a years commitment, its impractical and the agency will not get paid!
The agency does the same for the familes - they make sure the proper expectations are set for families. An Au Pair is expected to complete max 25 hours each week with regard to childcare and light household duties, with payment of (when I was working in the sector) approx 100-120 euros per week, and bed and board. If you are looking for any more than these hours then an Au Pair is not really for you - they want time to travel and see the country and learn the language, make friends etc. thats their reason for coming.
Nothing wrong with the websites, and I do mean that. You just need to have your wits about you and be able to read through some of the waffle, and you won't have the support the agencies offer, such as support for when things aren't going so well, Au Pair is homesick, meet ups for your Au Pair with others in your area or city, things to do and see, etc.
I've had some families have great success with Au Pairs from websites but do be on your guard. Set the right expectation for what you expect and need, and remember the guidelines. If its more than 25 hours, you dont need an Au Pair - you need a nanny or childminder.
Au Pairs are not to be seen as cheap labour. To tell you a story, I once had an Au Pair ring me who was placed on a farm with a family that knew the expectations. The Au Pair was being made to work on the farm for 10 hours a day every day, all for 100 euros a week with no days off. She was not given the job of minding the children at all. It really was awful, and the family were unapologetic about it even when we arrived at their home to remove the Au Pair. I've also had a family literally drop an 18 year old Au Pair at our offices with her bags and a note for us to say they no longer wanted her as the language barrier was too much. They hadn't even told the girl what was happening, she was so upset and confused. Be aware, they come here to learn english and not all have good language skills at the beginning, but they are quicker to learn when in a family environment.