gardai can prosecute you for speeding by travelling in a car behind you at the same speed, however they will need to have the car they were using calibrated for the purpose of a court case if you fight the case in order to prove the speed they and you were doing. If you lose the case cause the gardai has done his/her homework you will probally face the cost of having the car calibrated by the gardai for the purpose of the case and this would not be cheap you will also at this stage be getting double your penalty points and a much larger fine. If the garda car is an unmarked traffic camera car the camera acts as a speed detector and can be given in evidence once it is proved its in working order, be careful that when you look for this evidence that it doesnt show any dangerous driving as judge wont be impressed,