I am assuming that the Deputy is referring to Government Ministers and the Minister of State to the Government.
Official State cars are allocated to the Minister of each Department at the time of his-her appointment for use on official business. There are 47 cars in the Ministerial fleet of which 28 are permanently assigned to the Taoiseach, Tanaiste, Government Ministers, the Chief Whip and the Minister of State to the Government, the President, former President, former Taoisigh, Director of Public Prosecutions, Chief Justice, Attorney General, and the Ceann Comhairle. The remaining 19 vehicles are not specifically allocated but are used as replacements when vehicles are undergoing repairs, maintenance, etc.
The Garda authorities who are responsible for the allocation of Garda personnel have informed me that Garda drivers who are allocated to drive Government Ministers receive a Ministerial driver's allowance. When these drivers are absent due to leave, illness etc, they are relieved by drivers from Transport Details who receive a transport allowance together with overtime for any duty above their normal eight hour tours of duty.
In relation to civilian drivers selected by Ministers of State I have no function with regard to the procedures for the usage of such drivers.