I answered your taxi question on your other post/question on taxis.
But here is my answer. :-)
The golden rule on taxis is make sure the machine is on and read the guide lines for the rates (in the evenings the rates really go up) you can usually find these somewhere in the back. Lastly make sure the taxi driver has a taxi drivers licenses (I forget the legal wording) you want to make sure they are legally working for a taxi company (some of the taxis in Dublin don't have this). Mostly though the taxi drivers are honest when it comes to Ireland, I will say that the only flat rates that I saw came from getting a taxi through a Hotel, going out to the Airport.
Oh, lastly since Ireland had to open its job market to Poland and other Eastern European countries, you are now finding more and more Non-Irish taxi drivers. In some of the taxi lines you will see some Irish pass up taxis because of this, it doesn't mean the driver isn't good. I've taken Polish Taxi Drivers, the only thing is you may have to hand them the address for the place over just saying the name of the place.