As of January 1st 2004, new legislation was brought in that prohibits window tinting.
You are not allowed to tint your windscreen or front windows AT ALL. Anything forward of the B pillar (middle of the car) MUST NOT be tinted in anyway. You are however still allowed to perform any level of tinting on your back windows.
If you've tinted you windows before January that does not mean that you are safe, if the police believe that your tint causes reduced visibility they can issue you with an order to have it removed.
Why was this law brought in?
The reason this law was introduced was that there was a serious accident between a motorbike and a heavily modified car with very dark tints. It was believed that the root cause of the accident was that the driver couldn't see well enough through his tinted windows.