Spokeswoman Noreen Hegarty confirmed yesterday that police bosses were now taking further action.
"An internal investigation has been initiated into the activities of Mr Keenan, who was photographed holding a mobile phone while operating a police patrol vehicle."
It is illegal to operate a cellphone while driving a car, unless it is connected to a hands-free device.
Police are exempt from that law if they are using their phone in the execution of their duties.
Another senior officer said all police cars had been fitted with hands-free equipment "well before" cellphone legislation changed in 2009.
He said officers were told in no uncertain terms not to be seen using phones while driving. "If we do need to use it, we've been told to pull over and stop."